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=Embedded Polytropic Spheres=
=Embedded Polytropic Spheres=


[[Image:LSU_Structure_still.gif|90px|left]]  In a [[User:Tohline/SSC/Structure/Polytropes|separate discussion]] we have shown how to determine the structure of isolated polytropic spheres.  These are rather idealized stellar structures in which the pressure and density both drop to zero at the surface of the configuration.  Here we consider how the equilibrium radius of a polytropic configuration of a given <math>~M</math> and {{User:Tohline/Math/MP_PolytropicConstant}} is modified when it is embedded in an external medium of pressure <math>~P_e</math>.  We will begin by reviewing the general properties of embedded (and truncated) polytropes for a wide range of polytropic indexes, principally summarizing the published descriptions provided by [http://adsabs.harvard.edu/abs/1970MNRAS.151...81H Horedt (1970)], by  [http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth (1981)], by [http://adsabs.harvard.edu/abs/1981PASJ...33..273K Kimura (1981)], and by [http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler (1983)].  Then we will focus in more detail on polytropes of index {{User:Tohline/Math/MP_PolytropicIndex}} = 1 and {{User:Tohline/Math/MP_PolytropicIndex}} = 5 because their structures can be described by closed-form analytic expressions.
[[Image:LSU_Structure_still.gif|90px|left]]  In a [[User:Tohline/SSC/Structure/Polytropes|separate discussion]] we have shown how to determine the structure of isolated polytropic spheres.  These are rather idealized stellar structures in which the pressure and density both drop to zero at the surface of the configuration.  Here we consider how the equilibrium radius of a polytropic configuration of a given <math>~M</math> and {{User:Tohline/Math/MP_PolytropicConstant}} is modified when it is embedded in an external medium of pressure <math>~P_e</math>.  We will begin by reviewing the general properties of embedded (and truncated) polytropes for a wide range of polytropic indexes, principally summarizing the published descriptions provided by [http://adsabs.harvard.edu/abs/1970MNRAS.151...81H Horedt (1970)], by  [http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth (1981)], by [http://adsabs.harvard.edu/abs/1981PASJ...33..273K Kimura (1981a)], and by [http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler (1983)].  Then we will focus in more detail on polytropes of index {{User:Tohline/Math/MP_PolytropicIndex}} = 1 and {{User:Tohline/Math/MP_PolytropicIndex}} = 5 because their structures can be described by closed-form analytic expressions.




Line 354: Line 354:
===Kimura's Presentation===
===Kimura's Presentation===


At the same time Whitworth's work was being published, [http://adsabs.harvard.edu/abs/1981PASJ...33..273K Kimura (1981)] also published a derivation of the equations that define the equilibrium properties of embedded, pressure-truncated polytropic configurations.  (Note that an [http://adsabs.harvard.edu/abs/1981PASJ...33..749K erratum] has been published correcting typographical errors that appeared in a few equations of the original paper.)  When compared with, for example, Horedt's published work, Kimura's set of structural equations are a bit more difficult to digest because they include (a) an equation-of-state index that is different from the traditional ''polytropic index'' &#8212; specifically (see his equation 6),
At the same time Whitworth's work was being published, [http://adsabs.harvard.edu/abs/1981PASJ...33..273K Kimura (1981a)] also published a derivation of the equations that define the equilibrium properties of embedded, pressure-truncated polytropic configurations.  (Note that an [http://adsabs.harvard.edu/abs/1981PASJ...33..749K erratum] has been published correcting typographical errors that appear in a few equations of the original paper.)  When compared with, for example, [[User:Tohline/SSC/Structure/PolytropesEmbedded#Horedt.27s_Presentation|Horedt's published work]] &#8212; which Kimura references &#8212; Kimura's set of structural equations are a bit more difficult to digest because they include (a) an equation-of-state index that is different from the traditional ''polytropic index'' &#8212; specifically (see his equation 6),
<div align="center">
<div align="center">
<math>~\sigma \equiv (n+1)^{-1} \, ,</math>
<math>~\sigma \equiv (n+1)^{-1} \, </math>
</div>
</div>
in an effort to more naturally accommodate discussions of isothermal <math>~(n=\infty)</math> configurations; and (b) an additional integer index, <math>~m</math>, so that a single set of equations can be used to specify the structure of planar <math>~(m = 1)</math> and cylindrical <math>~(m=2)</math>, as well as spherical <math>~(m=3)</math> configurations.  Here, we will fix the value, <math>~m = 3</math>.  Kimura also chooses to express his structural solutions in terms of a dimensionless radius, <math>~\zeta</math>, instead of the traditional variable, <math>~\xi</math> &#8212; note that the two are related via the expression,
&#8212; which was Kimura's effort to more gracefully accommodate discussions of isothermal <math>~(n=\infty)</math> configurations; and (b) an additional integer index, <math>~m</math>, so that a single set of equations can be used to specify the structure of planar <math>~(m = 1)</math> and cylindrical <math>~(m=2)</math> as well as spherical <math>~(m=3)</math> configurations.  In the present context, we will fix the value to <math>~m = 3</math>.  Kimura also chose to express his structural solutions in terms of a dimensionless radius, <math>~\zeta</math>, instead of the traditional variable, <math>~\xi</math> &#8212; note that the two are related via the expression,
<div align="center">
<div align="center">
<math>~\zeta = (n+1)^{1/2} \xi \, ;</math>
<math>~\zeta = (n+1)^{1/2} \xi \, ;</math>
</div>
</div>
and usually in terms of a dimensionless gravitational potential, <math>~\phi</math>, instead of the traditional dimensionless enthalpy variable, <math>~\theta</math> &#8212; note that the two are related via the expression (see Kimura's equation 12),
and in terms of a dimensionless gravitational potential, <math>~\phi</math>, instead of the traditional dimensionless enthalpy variable, <math>~\theta_n</math> &#8212; note that the two are related via the expression (see Kimura's equation 12),
<div align="center">
<div align="center">
<math>~\phi = \sigma^{-1}(1 - \theta) \, .</math>
<math>~\phi = \sigma^{-1}(1 - \theta_n) \, .</math>
</div>
</div>
This allows us to acknowledge, as well, that,
Given this relationship, we note as well that,
<div align="center">
<div align="center">
<math>~\frac{d\phi}{d\zeta} = -\frac{d\theta}{d\xi} \cdot \biggl[ \sigma^{-1} \frac{d\xi}{d\zeta} \biggr] =  
<math>~\phi^' \equiv \frac{d\phi}{d\zeta} = -\frac{d\theta_n}{d\xi} \cdot \biggl[ \sigma^{-1} \frac{d\xi}{d\zeta} \biggr] =  
-\frac{d\theta}{d\xi} (n+1)^{1/2} \, .</math>
-\frac{d\theta_n}{d\xi} (n+1)^{1/2} \, .</math>
</div>
</div>


 
The set of equilibrium equations derived by [http://adsabs.harvard.edu/abs/1981PASJ...33..273K Kimura (1981a)] in what he identifies as "Paper I" &#8212; see especially his equations number (16) and (23) &#8212; are summarized most succinctly in Table 1 of  his "Paper II" ([http://adsabs.harvard.edu/abs/1981PASJ...33..299K Kimura 1981b]).  The equations he presents for "radial distance," "pressure," and "fractional mass within <math>~\tilde{\zeta}</math>" are, respectively,
 
===Stahler's Presentation===
Similarly, in Appendix B of his work, [http://adsabs.harvard.edu/abs/1983ApJ...268..165S Steven W. Stahler (1983)] states that the mass, <math>~M</math>, associated with the equilibrium radius, <math>~R_\mathrm{eq}</math>, of embedded polytropic spheres is,
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<table border="0" cellpadding="3">
Line 382: Line 379:
   <td align="right">
   <td align="right">
<math>
<math>
~M
~\frac{R_\mathrm{eq}}{R_\mathrm{Kimura}}
</math>
</math>
   </td>
   </td>
Line 390: Line 387:
   <td align="left">
   <td align="left">
<math>
<math>
M_\mathrm{SWS} \biggl( \frac{n^3}{4\pi} \biggr)^{1/2} \biggl\{ \theta_n^{(n-3)/2} \xi^2
\tilde\zeta = (n+1)^{1/2} \tilde\xi \, ,
\biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr\}_{\xi_e}
</math>
</math>
   </td>
   </td>
Line 399: Line 395:
   <td align="right">
   <td align="right">
<math>
<math>
~R_\mathrm{eq}
~\frac{P_\mathrm{e}}{P_\mathrm{Kimura}}
</math>
</math>
   </td>
   </td>
Line 407: Line 403:
   <td align="left">
   <td align="left">
<math>
<math>
R_\mathrm{SWS} \biggl( \frac{n}{4\pi} \biggr)^{1/2} \biggl\{ \xi \theta_n^{(n-1)/2} \biggr\}_{\xi_e}
\tilde\theta_n^{n+1}\, ,
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
</div>
where, from his equations (7) and (B3) we deduce,
<div align="center">
<math>M_\mathrm{SWS} =
\biggl( \frac{n+1}{nG} \biggr)^{3/2} K_n^{2n/(n+1)} P_\mathrm{e}^{(3-n)/[2(n+1)]} \, ,</math>
</div>
<div align="center">
<math>
R_\mathrm{SWS} = \biggl( \frac{n+1}{nG} \biggr)^{1/2} K_n^{n/(n+1)} P_\mathrm{e}^{(1-n)/[2(n+1)]} \, .
</math>
</div>
Notice that, via these two normalizations, Stahler chose to express <math>~R_\mathrm{eq}</math> and <math>~M</math> in terms of {{User:Tohline/Math/MP_PolytropicConstant}} and the applied external pressure, <math>~P_\mathrm{e}</math>.
===Reconciliation===
Here we demonstrate that Whitworth's and Stahler's presentations are equivalent to one another.  We begin by plugging Stahler's definition of <math>~M_\mathrm{SWS}</math> into his expression for <math>~M</math>, then inverting it to obtain an expression for <math>~P_\mathrm{e}</math> in terms of <math>~M</math> and {{User:Tohline/Math/MP_PolytropicConstant}}.
<div align="center">
<table border="0" cellpadding="3">


<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
<math>
~M
~\frac{M}{M_\mathrm{Kimura}}
</math>
</math>
   </td>
   </td>
Line 442: Line 419:
   <td align="left">
   <td align="left">
<math>
<math>
\biggl[ \frac{(n+1)^3}{4\pi G^3} \biggr]^{1/2} K_n^{2n/(n+1)} P_\mathrm{e}^{(3-n)/[2(n+1)]}  
\tilde\zeta^2 {\tilde\phi}^' = (n+1)^{3/2} \biggl[ - \xi^2 \frac{d\theta_n}{d\xi} \biggr]_{\tilde\xi} \, ,
\biggl\{ \theta_n^{(n-3)/2} \xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr\}_{\xi_e}
</math>
  </td>
</tr>
</table>
</div>
where, expressed in terms of the central pressure, <math>~p_*</math>, and the polytropic constant, <math>~K_n, ~[</math>note that, in Kimura's paper, <math>~H = K_n^{n/(n+1)}]</math>, the relevant normalization parameters are,
<div align="center">
<table border="0" cellpadding="3">
 
<tr>
  <td align="right">
<math>
~R_\mathrm{Kimura}
</math>
  </td>
  <td align="center">
<math>~\equiv~</math>
  </td>
  <td align="left">
<math>
(4\pi G)^{-1/2} H p_*^{\sigma - 1/2} =
(4\pi G)^{-1/2} K_n^{n/(n+1)} p_*^{(1-n)/[2(n+1)]} \, ,
</math>
</math>
   </td>
   </td>
Line 451: Line 449:
   <td align="right">
   <td align="right">
<math>
<math>
\Rightarrow ~~~~~ P_\mathrm{e}^{(3-n)}
~P_\mathrm{Kimura}
</math>
</math>
   </td>
   </td>
Line 459: Line 457:
   <td align="left">
   <td align="left">
<math>
<math>
\biggl[ \frac{4\pi G^3}{(n+1)^3} \biggr]^{(n+1)} K_n^{-4n}  M^{2(n+1)}
p_* \, ,
\biggl\{ \theta_n^{(n-3)/2} \xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr\}^{-2(n+1)}_{\xi_e}
</math>
</math>
   </td>
   </td>
Line 467: Line 464:
<tr>
<tr>
   <td align="right">
   <td align="right">
&nbsp;
<math>
~M_\mathrm{Kimura}
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
Line 474: Line 473:
   <td align="left">
   <td align="left">
<math>
<math>
\biggl[ \frac{4\pi G^3 M^2}{(n+1)^3} \biggr]^{(n+1)} K_n^{-4n}  
(4\pi G)^{-3/2} (4\pi) H^2 p_*^{2\sigma - 1/2} =
\biggl\{ \theta_n^{(3-n)} \xi^{-4} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}^{(n+1)}_{\xi_e}
(4\pi G^3)^{-1/2} K_n^{2n/(n+1)} p_*^{(3-n)/[2(n+1)]} \, .
</math>
</math>
   </td>
   </td>
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</table>
</table>
</div>
</div>
Alternatively, plugging Whitworth's definition of <math>~P_\mathrm{rf}</math> into his expression for <math>~P_\mathrm{e}</math> gives,
 
In order to compare Kimura's equilibrium expressions  for <math>~R_\mathrm{eq}</math> and <math>~P_e</math> with the corresponding expressions presented by [[User:Tohline/SSC/Structure/PolytropesEmbedded#Horedt.27s_Presentation|Horedt]] and by [[User:Tohline/SSC/Structure/PolytropesEmbedded#Whitworth.27s_Presentation|Whitworth]], we need to replace <math>~p_*</math> by <math>~M</math> in both expressions.  Inverting Kimura's expression for <math>~M</math>, we have,
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<table border="0" cellpadding="3">
Line 488: Line 488:
   <td align="right">
   <td align="right">
<math>
<math>
~P_\mathrm{e}^{(3-n)}
~p_*^{(3-n)/[2(n+1)]}
</math>
</math>
   </td>
   </td>
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   <td align="left">
   <td align="left">
<math>
<math>
2^{2(5n+1)} \biggl( \frac{\pi}{3^4 \cdot 5^3} \biggr)^{(n+1)} 2^{-8n} \cdot 3^{4(n+1)} \biggl(\frac{5}{n+1} \biggr)^{3(n+1)}
M (n+1)^{-3/2}( - \tilde\xi^2 \tilde\theta^' )^{-1} (4\pi G^3)^{1/2} K_n^{-2n/(n+1)} \, .
[ G^{3} M^{2} ]^{(n+1)} K_n^{-4n} \biggl\{  \theta_n^{(3-n)}\xi^{-4}
\biggl|\frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}_{\xi_e}^{(n+1)}  
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
</div>
Hence,
<div align="center">
<table border="0" cellpadding="3">


<tr>
<tr>
   <td align="right">
   <td align="right">
&nbsp;
<math>
~P_\mathrm{Kimura}
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
Line 512: Line 517:
   <td align="left">
   <td align="left">
<math>
<math>
2^{2(n+1)} \biggl[ \frac{\pi}{(n+1)^3} \biggr]^{(n+1)} [ G^{3} M^{2} ]^{(n+1)} K_n^{-4n} \biggl\{  \theta_n^{(3-n)}\xi^{-4}  
[ M (n+1)^{-3/2}( - \tilde\xi^2 \tilde\theta^' )^{-1} (4\pi G^3)^{1/2} K_n^{-2n/(n+1)} ]^{2(n+1)/(3-n)}
\biggl|\frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}_{\xi_e}^{(n+1)} \, .
\, ,
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
</div>
So Whitworth's and Stahler's relations for <math>~P_\mathrm{e}(M,K_n)</math> are, indeed, identical.  Similarly examining Stahler's expression for the equilibrium radius, we find,
<div align="center">
<table border="0" cellpadding="3">


<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
&nbsp;
~R_\mathrm{eq}
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
Line 534: Line 532:
   <td align="left">
   <td align="left">
<math>
<math>
\biggl( \frac{n+1}{4\pi G} \biggr)^{1/2} K_n^{n/(n+1)}
[ M^{-2} (n+1)^{3}( - \tilde\xi^2 \tilde\theta^' )^{2} (4\pi G^3)^{-1} K_n^{4n/(n+1)} ]^{(n+1)/(n-3)}  
\biggl[ \xi \theta_n^{(n-1)/2} \biggr]_{\xi_e} \biggl\{ P_\mathrm{e}^{1/(n+1)} \biggr\}^{(1-n)/2}
\, ,
</math>
</math>
   </td>
   </td>
Line 548: Line 546:
   </td>
   </td>
   <td align="left">
   <td align="left">
<math>
<math>~P_\mathrm{Horedt}
\biggl( \frac{n+1}{4\pi G} \biggr)^{1/2} K_n^{n/(n+1)}
[ ( - \tilde\xi^2 \tilde\theta^' )^{2} ]^{(n+1)/(n-3)}  
\biggl[ \xi \theta_n^{(n-1)/2} \biggr]_{\xi_e} \biggl\{
\biggl[ \frac{4\pi G^3 M^2}{(n+1)^3} \biggr] K_n^{-4n/(n+1)}
\biggl[ \theta_n^{(3-n)} \xi^{-4} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr]_{\xi_e}
\biggr\}^{(1-n)/[2(3-n)]}
</math>
</math>
   </td>
   </td>
</tr>
</tr>
<tr>
  <td align="right">
<math>\Rightarrow ~~~~~ P_e</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>~P_\mathrm{Horedt} ~\tilde\theta^{n+1}
( - \tilde\xi^2 \tilde\theta^' )^{2(n+1)/(n-3)}
\, ,
</math>
  </td>
</tr>
</table>
</div>
which matches Horedt's expression for <math>~P_e</math>.  Also after replacement we obtain,
<div align="center">
<table border="0" cellpadding="3">


<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
<math>
\Rightarrow ~~~~~ R_\mathrm{eq}^{(3-n)}
~R_\mathrm{Kimura}
</math>
</math>
   </td>
   </td>
Line 569: Line 583:
   <td align="left">
   <td align="left">
<math>
<math>
\biggl( \frac{n+1}{4\pi G} \biggr)^{(3-n)/2} K_n^{n(3-n)/(n+1)}  \xi_e^{3-n} \biggl\{
(4\pi G)^{-1/2} K^{n/(n+1)} [ M (n+1)^{-3/2}( - \tilde\xi^2 \tilde\theta^' )^{-1} (4\pi G^3)^{1/2} K_n^{-2n/(n+1)} ]^{(1-n)/(3-n)}
\biggl[ \frac{4\pi G^3 M^2}{(n+1)^3} \biggr]^{1/2} K_n^{-2n/(n+1)}  
\biggl[  \xi^{-2} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{-1} \biggr]_{\xi_e}
\biggr\}^{(1-n)}
</math>
</math>
   </td>
   </td>
Line 585: Line 596:
   </td>
   </td>
   <td align="left">
   <td align="left">
<math> (n+1)^{[(3-n)-3(1-n)]/2} (4\pi)^{[(n-3) +(1-n)]/2} G^{[(n-3)+3(1-n)]/2}
<math>
[K_n^{(3-n)+2(n-1)}]^{n/(n+1)}  \xi_e^{(3-n)+2(n-1)}
M^{(n-1)/(n-3)} ( - \tilde\xi^2 \tilde\theta^' )^{(1-n)/(n-3)} (n+1)^{3(1-n)/2(n-3)}  
M^{(1-n)} \biggl| \frac{d\theta}{d\xi} \biggr|^{(n-1)}_{\xi_e}
(4\pi)^{[(1-n)-(3-n)]/[2(3-n)]} G^{[3(1-n)- (3-n)]/[2(3-n)]} [ K_n^{n(3-n)-2n(1-n)} ]^{1/[(n+1)(3-n)]}
</math>
</math>
   </td>
   </td>
Line 600: Line 611:
   </td>
   </td>
   <td align="left">
   <td align="left">
<math> (n+1)^{n} (4\pi)^{-1} G^{-n} K_n^n M^{(1-n)}  
<math>
\biggl[ \xi^{(n+1)} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{(n-1)}\biggr]_{\xi_e} \, .
M^{(n-1)/(n-3)} ( - \tilde\xi^2 \tilde\theta^' )^{(1-n)/(n-3)} (n+1)^{3(1-n)/2(n-3)}  
(4\pi)^{1/(n-3)} G^{n/(n-3)} K_n^{-n/(n-3)}
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
 
</div>
<tr>
And Whitworth's expression becomes,
<div align="center">
<table border="0" cellpadding="3">
 
<tr>
   <td align="right">
   <td align="right">
<math>
<math>~\Rightarrow ~~~~~ R_\mathrm{eq}</math>
~R_\mathrm{eq}^{(3-n)}
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
Line 621: Line 626:
   </td>
   </td>
   <td align="left">
   <td align="left">
<math>
<math>\tilde\xi( - \tilde\xi^2 \tilde\theta^' )^{(1-n)/(n-3)}
2^{-2(n+1)} \pi^{-1} 3^{n+1} \cdot 5^{n} K_n^n G^{-n} M^{1-n}
(n+1)^{[3(1-n)+(n-3)]/2(n-3)}  
\biggl[ \frac{4(n+1)}{5} \biggr]^{n} \biggl(\frac{\xi_e}{3} \biggr)^{(n+1)}  
\biggl[ 4\pi \biggl( \frac{G}{K_n} \biggr)^{n} M^{(n-1)} \biggr]^{1/(n-3)}
\biggl|\frac{d\theta_n}{d\xi} \biggr|^{(n-1)}_{\xi_e}  
</math>
</math>
   </td>
   </td>
Line 637: Line 641:
   </td>
   </td>
   <td align="left">
   <td align="left">
<math>
<math>  
(n+1)^n (4\pi)^{-1} K_n^n G^{-n} M^{1-n} \xi_e^{(n+1)}
R_\mathrm{Horedt}~ \tilde\xi( - \tilde\xi^2 \tilde\theta^' )^{(1-n)/(n-3)} \, ,
\biggl|\frac{d\theta_n}{d\xi} \biggr|^{(n-1)}_{\xi_e} \, .
</math>
</math>
   </td>
   </td>
Line 645: Line 648:
</table>
</table>
</div>
</div>
Hence, Stahler's equilibrium radius, <math>~R_\mathrm{eq}</math>, exactly matches Whitworth's <math>~R_\mathrm{eq}</math>.
which exactly matches Horedt's expression for <math>~R_\mathrm{eq}</math>.
 
===Summary===


<div align="center">
===Stahler's Presentation===
<table border="1" cellpadding="8" width="95%">
Similarly, in Appendix B of his work, [http://adsabs.harvard.edu/abs/1983ApJ...268..165S Steven W. Stahler (1983)] states that the mass, <math>~M</math>, associated with the equilibrium radius, <math>~R_\mathrm{eq}</math>, of embedded polytropic spheres is,
<tr><td align="left">
Once the function, <math>~\theta_n(\xi)</math>, and its first derivative with respect to the dimensionless radial coordinate, <math>~d\theta_n/d\xi</math>, are obtained via a solution of the Lane-Emden equation, the equilibrium radius, <math>~R_\mathrm{eq}</math>, and total mass, <math>~M</math>, of a pressure-bounded polytrope can be expressed in terms of Stahler's normalizations as follows:
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<table border="0" cellpadding="3">
Line 659: Line 658:
   <td align="right">
   <td align="right">
<math>
<math>
\frac{R_\mathrm{eq}}{R_\mathrm{SWS}}
~M
</math>
</math>
   </td>
   </td>
Line 667: Line 666:
   <td align="left">
   <td align="left">
<math>
<math>
\biggl( \frac{n}{4\pi} \biggr)^{1/2}\biggl[ \xi \theta_n^{(n-1)/2} \biggr]_{\xi_e} \, ,
M_\mathrm{SWS} \biggl( \frac{n^3}{4\pi} \biggr)^{1/2} \biggl\{ \theta_n^{(n-3)/2} \xi^2
\biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr\}_{\xi_e}
</math>
</math>
   </td>
   </td>
Line 675: Line 675:
   <td align="right">
   <td align="right">
<math>
<math>
\frac{M}{M_\mathrm{SWS}}
~R_\mathrm{eq}
</math>
</math>
   </td>
   </td>
Line 683: Line 683:
   <td align="left">
   <td align="left">
<math>
<math>
\biggl( \frac{n^3}{4\pi} \biggr)^{1/2} p_a^{(n-3)/[2(n+1)]} \, ,
R_\mathrm{SWS} \biggl( \frac{n}{4\pi} \biggr)^{1/2} \biggl\{ \xi \theta_n^{(n-1)/2} \biggr\}_{\xi_e}
</math>
</math>
   </td>
   </td>
Line 689: Line 689:
</table>
</table>
</div>
</div>
where,
where, from his equations (7) and (B3) we deduce,
<div align="center">
<math>M_\mathrm{SWS} =
\biggl( \frac{n+1}{nG} \biggr)^{3/2} K_n^{2n/(n+1)} P_\mathrm{e}^{(3-n)/[2(n+1)]} \, ,</math>
</div>
<div align="center">
<math>
R_\mathrm{SWS} = \biggl( \frac{n+1}{nG} \biggr)^{1/2} K_n^{n/(n+1)} P_\mathrm{e}^{(1-n)/[2(n+1)]} \, .
</math>
</div>
Notice that, via these two normalizations, Stahler chose to express <math>~R_\mathrm{eq}</math> and <math>~M</math> in terms of {{User:Tohline/Math/MP_PolytropicConstant}} and the applied external pressure, <math>~P_\mathrm{e}</math>.
 
===Reconciliation===
Here we demonstrate that Whitworth's and Stahler's presentations are equivalent to one another.  We begin by plugging Stahler's definition of <math>~M_\mathrm{SWS}</math> into his expression for <math>~M</math>, then inverting it to obtain an expression for <math>~P_\mathrm{e}</math> in terms of <math>~M</math> and {{User:Tohline/Math/MP_PolytropicConstant}}.


<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<table border="0" cellpadding="3">
<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
<math>
~p_a
~M
</math>
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
<math>~\equiv~</math>
<math>~=~</math>
   </td>
   </td>
   <td align="left">
   <td align="left">
<math>
<math>
\biggl[ \theta^{(n-3)/2}_n \xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr]_{\xi_e}^{2(n+1)/(n-3)}
\biggl[ \frac{(n+1)^3}{4\pi G^3} \biggr]^{1/2} K_n^{2n/(n+1)} P_\mathrm{e}^{(3-n)/[2(n+1)]}  
= \theta_n^{(n+1)} \biggl(\xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr)_{\xi_e}^{2(n+1)/(n-3)} \, .
\biggl\{ \theta_n^{(n-3)/2} \xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr\}_{\xi_e}
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
 
</div>
<tr>
Then, the external pressure, expressed in terms of Whitworth's normalization, is,
<div align="center">
<table border="0" cellpadding="3">
<tr>
   <td align="right">
   <td align="right">
<math>
<math>
\frac{P_\mathrm{e}}{P_\mathrm{rf}}  
\Rightarrow ~~~~~ P_\mathrm{e}^{(3-n)}
</math>
</math>
   </td>
   </td>
Line 725: Line 735:
   <td align="left">
   <td align="left">
<math>
<math>
~2^{8n/(n-3)} \biggl[ \frac{(n+1)^3}{3^4\cdot 5^3}\biggr]^{(n+1)/(n-3)} p_a \, ;
\biggl[ \frac{4\pi G^3}{(n+1)^3} \biggr]^{(n+1)} K_n^{-4n} M^{2(n+1)}
\biggl\{ \theta_n^{(n-3)/2} \xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr\}^{-2(n+1)}_{\xi_e}
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
 
</div>
and the conversion from Stahler's normalization to Whitworth's normalization of the radius is achieved via the expression,
<div align="center">
<table border="0" cellpadding="3">
<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
&nbsp;
\frac{R_\mathrm{SWS}}{R_\mathrm{rf}}
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
Line 745: Line 750:
   <td align="left">
   <td align="left">
<math>
<math>
~\biggl[ \frac{3^{(n+1)}}{2^{(n+3)}} \biggl( \frac{5}{n+1} \biggr)^n \biggr]^{1/(n-3)} \biggl( \frac{\pi}{n} \biggr)^{1/2} p_a^{(1-n)/[2(n+1)]} \, .
\biggl[ \frac{4\pi G^3 M^2}{(n+1)^3} \biggr]^{(n+1)} K_n^{-4n}  
\biggl\{ \theta_n^{(3-n)} \xi^{-4} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}^{(n+1)}_{\xi_e}
</math>
</math>
   </td>
   </td>
Line 751: Line 757:
</table>
</table>
</div>
</div>
Alternatively, plugging Whitworth's definition of <math>~P_\mathrm{rf}</math> into his expression for <math>~P_\mathrm{e}</math> gives,
<div align="center">
<table border="0" cellpadding="3">


</td></tr>
<tr>
</table>
  <td align="right">
</div>
 
=={{User:Tohline/Math/MP_PolytropicIndex}} = 1 Polytrope ==
Drawing from the [[User:Tohline/SSC/Structure/Polytropes|earlier discussion of isolated polytropes]], we will reference various radial locations within the spherical configuration by the dimensionless radius,
<div align="center">
<math>
<math>
\xi \equiv \frac{r}{a_\mathrm{n=1}} ,
~P_\mathrm{e}^{(3-n)}
</math>
</math>
</div>
  </td>
where,
  <td align="center">
<div align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
<math>
a_\mathrm{n=1} \equiv \biggl[\frac{1}{4\pi G}~ \biggl( \frac{H_c}{\rho_c} \biggr)_{n=1}\biggr]^{1/2} = \biggl[\frac{K}{2\pi G} \biggr]^{1/2} \, .
2^{2(5n+1)} \biggl( \frac{\pi}{3^4 \cdot 5^3} \biggr)^{(n+1)} 2^{-8n} \cdot 3^{4(n+1)} \biggl(\frac{5}{n+1} \biggr)^{3(n+1)}
[ G^{3} M^{2} ]^{(n+1)} K_n^{-4n} \biggl\{  \theta_n^{(3-n)}\xi^{-4}  
\biggl|\frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}_{\xi_e}^{(n+1)}
</math>
</math>
</div>
  </td>
The solution to the Lane-Emden equation for <math>~n = 1</math> is,
</tr>
<div align="center">
 
<table border="0" cellpadding="3">
<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
&nbsp;
~\theta_1
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
Line 783: Line 788:
   <td align="left">
   <td align="left">
<math>
<math>
\frac{\sin\xi}{\xi} \, ,
2^{2(n+1)} \biggl[ \frac{\pi}{(n+1)^3} \biggr]^{(n+1)} [ G^{3} M^{2} ]^{(n+1)} K_n^{-4n} \biggl\{  \theta_n^{(3-n)}\xi^{-4}  
\biggl|\frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}_{\xi_e}^{(n+1)} \, .
</math>
</math>
   </td>
   </td>
Line 789: Line 795:
</table>
</table>
</div>
</div>
hence,
So Whitworth's and Stahler's relations for <math>~P_\mathrm{e}(M,K_n)</math> are, indeed, identical.  Similarly examining Stahler's expression for the equilibrium radius, we find,
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<table border="0" cellpadding="3">
<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
<math>
\frac{d\theta_1}{d\xi}
~R_\mathrm{eq}
</math>
</math>
   </td>
   </td>
Line 803: Line 810:
   <td align="left">
   <td align="left">
<math>
<math>
\frac{\cos\xi}{\xi} - \frac{\sin\xi}{\xi^2} \, .
\biggl( \frac{n+1}{4\pi G} \biggr)^{1/2} K_n^{n/(n+1)} 
\biggl[ \xi \theta_n^{(n-1)/2} \biggr]_{\xi_e} \biggl\{ P_\mathrm{e}^{1/(n+1)} \biggr\}^{(1-n)/2}
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
</div>


<font color="darkblue">
<tr>
===Review===
  <td align="right">
</font>
&nbsp;
 
  </td>
Again, from the [[User:Tohline/SSC/Structure/Polytropes|earlier discussion]], we can describe the properties of an isolated, spherical {{User:Tohline/Math/MP_PolytropicIndex}} = 1 polytrope as follows:
  <td align="center">
* <font color="red">Mass</font>: 
<math>~=~</math>
: In terms of the central density, <math>\rho_c</math>, and {{User:Tohline/Math/MP_PolytropicConstant}}, the total mass is,
  </td>
<div align="center">
  <td align="left">
<math>M = \frac{4}{\pi} \rho_c (\pi a_{n=1})^3 = 4\pi^2 \rho_c \biggl[\frac{K}{2\pi G} \biggr]^{3/2} = \rho_c \biggl[\frac{2\pi K^3}{G^3} \biggr]^{1/2}</math> ;
<math>
</div>
\biggl( \frac{n+1}{4\pi G} \biggr)^{1/2} K_n^{n/(n+1)} 
\biggl[ \xi \theta_n^{(n-1)/2} \biggr]_{\xi_e} \biggl\{
\biggl[ \frac{4\pi G^3 M^2}{(n+1)^3} \biggr] K_n^{-4n/(n+1)}
\biggl[ \theta_n^{(3-n)} \xi^{-4} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr]_{\xi_e}
\biggr\}^{(1-n)/[2(3-n)]}
</math>
  </td>
</tr>


: and, expressed as a function of <math>M</math>, the mass that lies interior to the dimensionless radius <math>\xi</math> is,
<tr>
<div align="center">
  <td align="right">
<math>\frac{M_\xi}{M} = \frac{1}{\pi} \biggl[ \sin\xi - \xi\cos\xi \biggr] \, ,~~~~~~\mathrm{for}~\pi \ge \xi \ge 0 \, .</math>  
<math>
</div>
\Rightarrow ~~~~~ R_\mathrm{eq}^{(3-n)}
: Hence,
</math>
<div align="center">
  </td>
<math>M_\xi = \rho_c \biggl[\frac{2K^3}{\pi G^3} \biggr]^{1/2} \biggl[ \sin\xi - \xi\cos\xi \biggr] \, .</math>
  <td align="center">
</div>
<math>~=~</math>
 
  </td>
* <font color="red">Pressure</font>:
  <td align="left">
: The central pressure of the configuration is,
<math>
<div align="center">
\biggl( \frac{n+1}{4\pi G} \biggr)^{(3-n)/2} K_n^{n(3-n)/(n+1)}  \xi_e^{3-n} \biggl\{
<math>P_c = \biggl[ \frac{G^3}{2\pi} \rho_c^4 M^2 \biggr]^{1/3} = \biggl[ \frac{G^3}{2\pi} \rho_c^6 \biggl(\frac{2\pi K^3}{G^3} \biggr) \biggr]^{1/3} = K\rho_c^2</math> ;
\biggl[ \frac{4\pi G^3 M^2}{(n+1)^3} \biggr]^{1/2} K_n^{-2n/(n+1)}  
</div>
\biggl[ \xi^{-2} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{-1} \biggr]_{\xi_e}
\biggr\}^{(1-n)}
</math>
  </td>
</tr>


: and, expressed in terms of the central pressure <math>P_c</math>, the variation with radius of the pressure is,
<tr>
<div align="center">
  <td align="right">
<math>P_\xi= P_c \biggl[ \frac{\sin\xi}{\xi} \biggr]^2</math> .
&nbsp;
</div>
  </td>
: Hence,
  <td align="center">
<div align="center">
<math>~=~</math>
<math>P_\xi= K\rho_c^2 \biggl[ \frac{\sin\xi}{\xi} \biggr]^2</math> .
  </td>
</div>
  <td align="left">
 
<math> (n+1)^{[(3-n)-3(1-n)]/2} (4\pi)^{[(n-3) +(1-n)]/2} G^{[(n-3)+3(1-n)]/2}
===Extension to Bounded Sphere===
[K_n^{(3-n)+2(n-1)}]^{n/(n+1)}  \xi_e^{(3-n)+2(n-1)}
Eliminating <math>\rho_c</math> between the last expression for <math>M_\xi</math> and the last expression for <math>P_\xi</math> gives,
M^{(1-n)} \biggl| \frac{d\theta}{d\xi} \biggr|^{(n-1)}_{\xi_e}
<div align="center">
<math>P_\xi= \biggl[\frac{\pi}{2} \cdot \frac{G^3 M_\xi^2}{K^2} \biggr] \biggl[ \frac{\sin\xi}{\xi(\sin\xi - \xi \cos\xi )} \biggr]^2</math> .
</div>
Now, if we rip off an outer layer of the star down to some dimensionless radius <math>\xi_e < \pi</math>, the interior of the configuration that remains &#8212; containing mass <math>M_{\xi_e}</math> &#8212; should remain in equilibrium if we impose the appropriate amount of externally applied pressure <math>P_e = P_{\xi_e} </math> at that radius.  (This will work only for spherically symmetric configurations, as the gravitation acceleration at any location only depends on the mass contained inside that radius.) If we rescale our solution such that the mass enclosed within <math>\xi_e</math> is the original total mass <math>M</math>, then the pressure that must be imposed by the external medium in which the configuration is embedded is,
<div align="center">
<math>P_e= \biggl[\frac{\pi}{2} \cdot \frac{G^3 M^2}{K^2} \biggr] \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2</math> .
</div>
The associated equilibrium radius of this pressure-confined configuration is,
<div align="center">
<math>
R_\mathrm{eq} = \xi_e a_\mathrm{n=1} = \biggl[ \frac{K}{2\pi G} \biggr]^{1/2} \xi_e  
</math>
</math>
</div>
  </td>
</tr>


====Overlap with Whitworth's Presentation====
<tr>
The solid green curve in the two top panels of Figure 1 shows how <math>R_\mathrm{eq}</math> varies with the applied external pressure <math>P_e</math> for this pressure-bounded <math>~n=1</math> model sequence.  In the top-right panel, following the lead of [http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth] (1981, MNRAS, 195, 967) &#8212; for clarification, read the [[User:Tohline/SSC/Structure/PolytropesASIDE1|accompanying ASIDE]] &#8212; these two quantities have been respectively normalized (or, "referenced") to,
  <td align="right">
<div align="center">
&nbsp;
<math>
  </td>
R_\mathrm{rf}\biggr|_\mathrm{n=1} \equiv \biggl( \frac{3^2 \cdot 5}{2^4 \pi} \biggr)^{1/2} \biggl(\frac{K}{G}\biggr)^{1/2} ~~~\Rightarrow ~~~ \frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl( \frac{2^3}{3^2 \cdot 5} \biggr)^{1/2} \xi_e \, ;
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math> (n+1)^{n} (4\pi)^{-1} G^{-n} K_n^n M^{(1-n)}  
\biggl[ \xi^{(n+1)} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{(n-1)}\biggr]_{\xi_e} \, .
</math>
</math>
  </td>
</tr>
</table>
</div>
</div>
and,
And Whitworth's expression becomes,
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
<math>
P_\mathrm{rf}\biggr|_\mathrm{n=1} \equiv \frac{2^6 \pi}{3^4 \cdot 5^3}  \biggl(\frac{G^3 M^2}{K^2}\biggr) ~~~\Rightarrow ~~~ \frac{P_e}{P_\mathrm{rf}} = \biggl( \frac{3^4 \cdot 5^3}{2^7} \biggr) \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2 \, .
~R_\mathrm{eq}^{(3-n)}
</math>
</math>
</div>
Note that this pair of mathematical expressions has been recorded to the immediate right of Whitworth's name in our [[User:Tohline/SSC/Structure/PolytropesEmbedded#n1Summary|<math>~n=1</math> summary table]].
In the top-left panel of Figure 1, the solid green curve shows the identical sequence, but plotted as <math>~\log(p_a)</math> versus <math>~log(r_a)</math>, for easier comparison with Horedt's work.  The pair of mathematical expressions defining <math>~r_a(\xi_e)</math> and <math>~p_a(\xi_e)</math> has been recorded to the immediate right of Horedt's name in the same [[User:Tohline/SSC/Structure/PolytropesEmbedded#n1Summary|summary table]].
<span id="WhitworthFig1b">
<div align="center">
<table border="2" cellpadding="8" width="85%">
<tr>
  <td align="center" colspan="2">
'''Figure 1:''' <font color="darkblue">Equilibrium R-P Diagram </font>
   </td>
   </td>
</tr>
   <td align="center">
<tr>
<math>~=~</math>
   <td align="left" colspan="2">
  </td>
All of the plots shown in this figure illustrate how the equilibrium radius of a pressure-bounded polytrope varies with the applied external pressure.  In the right-hand column, the log-log plots display a normalized <math>~P_e</math> along the horizontal axis and a normalized <math>~R_\mathrm{eq}</math> along the horizontal axis; in the left-hand column, these axes are flipped, and a different normalization is used.  One primary intent of all the diagrams is to show that, for polytropic sequences having <math>~n > 3</math> (or, equivalently, sequences having <math>\gamma_g \equiv 1 + 1/n < 4/3),</math> no equilibrium models exist above some limiting external pressure.
  <td align="left">
<math>
2^{-2(n+1)} \pi^{-1} 3^{n+1} \cdot 5^{n} K_n^n G^{-n} M^{1-n}
\biggl[ \frac{4(n+1)}{5} \biggr]^{n} \biggl(\frac{\xi_e}{3} \biggr)^{(n+1)}
\biggl|\frac{d\theta_n}{d\xi} \biggr|^{(n-1)}_{\xi_e}
</math>
   </td>
   </td>
</tr>
</tr>


<tr>
<tr>
   <td align="center" bgcolor="white">
   <td align="right">
[[File:HoredtPlot2.png|350px|center|To be compared with Horedt (1970)]]
&nbsp;
   </td>
   </td>
   <td align="center" bgcolor="white">
   <td align="center">
[[File:WhitworthPlot2.png|400px|center|To be compared with Whitworth (1981)]]
<math>~=~</math>
   </td>
   </td>
</tr>
   <td align="left">
 
<math>
<tr>
(n+1)^n (4\pi)^{-1} K_n^n G^{-n} M^{1-n} \xi_e^{(n+1)}
   <td align="center" bgcolor="white">
\biggl|\frac{d\theta_n}{d\xi} \biggr|^{(n-1)}_{\xi_e} \, .
[[File:Horedt_PRdiagram0.png|400px|center|Horedt (1970) Figure 1]]
</math>
  </td>
  <td align="center" bgcolor="white">
[[File:WhitworthFig1bCopy.jpg|450px|center|Whitworth (1981) Figure 1b]]
   </td>
   </td>
</tr>
</tr>
</table>
</div>
Hence, Stahler's equilibrium radius, <math>~R_\mathrm{eq}</math>, exactly matches Whitworth's <math>~R_\mathrm{eq}</math>.


<tr>
===Summary===
  <td align="center" bgcolor="white">
[[File:Horedt_EmbeddedPolytrope.png|350px|center|Horedt (1970) Title Page]]
  </td>
  <td align="center" bgcolor="white">
[[File:Whitworth1981TitlePage0.png|260px|center|Whitworth (1981) Title Page]]
  </td>
</tr>
 
<tr>
  <td align="left" colspan="2">
''Bottom Left'' [reproduction of Figure 1 from [http://adsabs.harvard.edu/abs/1970MNRAS.151...81H Horedt (1970)]]:  All three displayed sequences &#8212; <math>~n=4</math> (<math>~\gamma_g = 1.25</math>), <math>~n=5</math> (<math>~\gamma_g = 1.20</math>), and <math>~n=\infty</math> (<math>~\gamma_g = 1</math>, hence, isothermal) &#8212; exhibit an upper limit for the bounding pressure.  Each sequence displays two segments &#8212; a solid segment and a dashed segment &#8212; indicating that, below the maximum allowed value of <math>~P_e</math>, it is possible to construct two (or more) equilibrium configurations; models lying along the solid segment of each displayed curve are expected to be dynamically stable while models lying along the dashed segments are unstable.
 
''Bottom Right'' [reproduction of Figure 1b from [http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth (1981)]]:  Model sequences are shown for five different effective adiabatic indexes &#8212; <math>~\gamma_g = 1/3,~ 2/3,~ 1,~ 4/3,</math> and <math>~ 5/3</math> &#8212; corresponding, respectively, to polytropic indexes <math>~n = -2/3, -1/3, \infty, ~3/2, </math> and <math>~3</math>.  The three sequences having <math>~\gamma_g < 4/3</math> exhibit an upper limit for the bounding pressure.  Both the stable (solid) curve segment and the unstable (dashed) curve segment are drawn for the isothermal <math>~(\gamma_g = 1)</math> sequence, which is also displayed (as the <math>~n=\infty</math> sequence) in Horedt's diagram.
 
''Top'':  Plots that we have generated for direct comparison with Horedt's diagram (''left'') and with Whitworth's diagram (''right'').  Both plots display only the two sequences that are analytically prescribable:  <math>~n=1</math> (<math>~\gamma_g = 2</math>) and <math>~n=5</math> (<math>~\gamma_g = 1.20</math>).  Along the <math>~n=1</math> (green) sequence, stable equilibrium models can be constructed for all values of <math>~P_e</math>.  Along the <math>~n=5</math> sequence, equilibrium models only exist for values of <math>~P_e</math> less than the critical value, <math>~P_\mathrm{max} = (2^5\cdot 3^9/5^9) P_\mathrm{rf} = (3^{12}/2^{24}) P_\mathrm{Horedt}</math>; below this critical pressure, the sequence has two branches denoted by blue diamonds (stable models) and red squares (unstable models).
  </td>
</tr>
</table>
</div>
</span>


====Overlap with Stahler's Presentation====
We can invert the above expression for <math>~P_e(K,M)</math> to obtain the following expression for <math>~M(K,P_e)</math>:
<div align="center">
<div align="center">
<math>~M= K \biggl[\frac{2}{\pi} \cdot \frac{P_e}{G^3} \biggr]^{1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr]</math> .
<table border="1" cellpadding="8" width="95%">
</div>
<tr><td align="left">
If, following Stahler's lead, we normalize this expression by <math>~M_\mathrm{SWS}</math> (evaluated for <math>~n=1</math>) and we normalize the above expression for <math>~R_\mathrm{eq}</math> by <math>~R_\mathrm{SWS}</math> (evaluated for <math>~n=1</math>), we obtain,
Once the function, <math>~\theta_n(\xi)</math>, and its first derivative with respect to the dimensionless radial coordinate, <math>~d\theta_n/d\xi</math>, are obtained via a solution of the Lane-Emden equation, the equilibrium radius, <math>~R_\mathrm{eq}</math>, and total mass, <math>~M</math>, of a pressure-bounded polytrope can be expressed in terms of Stahler's normalizations as follows:
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<table border="0" cellpadding="3">
Line 945: Line 935:
   <td align="right">
   <td align="right">
<math>
<math>
\frac{M}{M_\mathrm{SWS}}
\frac{R_\mathrm{eq}}{R_\mathrm{SWS}}
</math>
</math>
   </td>
   </td>
Line 953: Line 943:
   <td align="left">
   <td align="left">
<math>
<math>
K \biggl[\frac{2}{\pi} \cdot \frac{P_e}{G^3} \biggr]^{1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr]
\biggl( \frac{n}{4\pi} \biggr)^{1/2}\biggl[ \xi \theta_n^{(n-1)/2} \biggr]_{\xi_e} \, ,
\biggl[ \biggl( \frac{G}{2} \biggr)^{3/2} K^{-1} P_\mathrm{ex}^{-1/2}  \biggr]
</math>
</math>
   </td>
   </td>
Line 961: Line 950:
<tr>
<tr>
   <td align="right">
   <td align="right">
&nbsp;
<math>
\frac{M}{M_\mathrm{SWS}}
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
Line 968: Line 959:
   <td align="left">
   <td align="left">
<math>
<math>
(4\pi)^{-1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr] \, ,
\biggl( \frac{n^3}{4\pi} \biggr)^{1/2} p_a^{(n-3)/[2(n+1)]} \, ,
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
</div>
where,


<div align="center">
<table border="0" cellpadding="3">
<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
<math>
\frac{R_\mathrm{eq}}{R_\mathrm{SWS}}
~p_a
</math>
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
<math>~=~</math>
<math>~\equiv~</math>
   </td>
   </td>
   <td align="left">
   <td align="left">
<math>
<math>
\biggl[ \frac{K}{2\pi G} \biggr]^{1/2} \xi_e \biggl[ \frac{G}{2K} \biggr]^{1/2} =
\biggl[ \theta^{(n-3)/2}_n \xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr]_{\xi_e}^{2(n+1)/(n-3)}
(4\pi)^{-1/2} \xi_e \, .
= \theta_n^{(n+1)} \biggl(\xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr)_{\xi_e}^{2(n+1)/(n-3)} \, .
</math>
</math>
   </td>
   </td>
Line 991: Line 987:
</table>
</table>
</div>
</div>
 
Then, the external pressure, expressed in terms of Whitworth's normalization, is,
<span id="Stahler1983Fig17">
<div align="center">
<div align="center">
<table border="2" cellpadding="8">
<table border="0" cellpadding="3">
<tr>
<tr>
   <td align="center" colspan="2">
   <td align="right">
'''Figure 2:''' <font color="darkblue">Equilibrium Mass-Radius Diagram </font>  
<math>
\frac{P_\mathrm{e}}{P_\mathrm{rf}}
</math>
   </td>
   </td>
</tr>
   <td align="center">
<tr>
<math>~=~</math>
   <td align="center" bgcolor="white">
[[File:Stahler1983TitlePage0.png|300px|center|Stahler (1983) Title Page]]
   </td>
   </td>
   <td valign="top" width=350 rowspan="3">
   <td align="left">
''Top:'' A slightly edited reproduction of Figure 17 in association with Appendix B of  [http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Stahler] (1983, ApJ, 268, 165).  Stahler's figure caption reads, in part, "Mass-radius relation for bounded polytropes (schematic).  Each curve is labeled by the appropriate value or range" of {{User:Tohline/Math/MP_PolytropicIndex}} &hellip; "As the cloud density increases from unity, all curves leave the origin with the same slope &hellip;" 
<math>
 
~2^{8n/(n-3)} \biggl[ \frac{(n+1)^3}{3^4\cdot 5^3}\biggr]^{(n+1)/(n-3)} p_a \, ;
 
</math>
''Bottom:''  Curves depict the exact, analytically derived mass-radius relationship for truncated <math>~n = 1</math> (purple squares) and <math>~n = 5</math> (blue diamonds) polytropes that are embedded in an external medium of pressure <math>~P_e</math>; the relevant mathematical expressions are presented to the immediate right of Stahler's name in, respectively, our [[User:Tohline/SSC/Structure/PolytropesEmbedded#n1Summary|<math>~n=1</math> summary table]] and our [[User:Tohline/SSC/Structure/PolytropesEmbedded#n5Summary|<math>~n=5</math> summary table]].  As the dimensionless truncation radius, <math>~\xi_e</math>, increases steadily from zero, both curves exhibit very similar behavior up to <math>~M_n \equiv M/M_\mathrm{SWS} \approx 0.5</math>; thereafter the normalized mass and normalized radius continue to steadily increase along the <math>~n = 1</math> sequence, but the <math>~n = 5</math> sequence eventually bends back on itself, returning to the origin as <math>~\xi_e \rightarrow \infty</math>. 
 
 
''Comparison:'' The monotonic <math>P-R</math> behavior of the analytically derived solution for {{User:Tohline/Math/MP_PolytropicIndex}} = 1 <math>(\gamma_g = 2)</math>, shown above, is consistent with the behavior of the numerically derived solutions presented by Whitworth for slightly lower values of <math>\gamma_g</math> = 5/3 and 4/3.  The analytically derived solution for {{User:Tohline/Math/MP_PolytropicIndex}} = 5 <math>(\gamma_g = 6/5)</math> shows that, above some limiting pressure, no equilibrium configuration exists; this is consistent with the behavior of the numerically derived solutions presented by Whitworth for all values of <math>\gamma_g < 4/3 \, .</math>
</td>
</tr>
 
<tr>
  <td align="center" bgcolor="white">
[[File:Stahler_MRdiagram1.png|300px|center|Stahler (1983) Figure 17 (edited)]]
  </td>
</tr>
 
<tr>
  <td align="center" bgcolor="white">
[[File:Stahler1983Comparison.png|300px|center|To be compared with Stahler (1983)]]
   </td>
   </td>
</tr>
</tr>
</table>
</table>
</div>
</div>
</span>
and the conversion from Stahler's normalization to Whitworth's normalization of the radius is achieved via the expression,
 
 
===Tabular Summary (n=1) ===
<span id="n1Summary">
<div align="center">
<div align="center">
<table border="1" cellpadding="8" width="95%">
<tr>
  <th align="center" colspan="3">
Properties of <math>~n=1</math> Polytropes Embedded in an External Medium of Pressure <math>~P_e</math>
<br>
(and, accordingly, truncated at radius <math>~\xi_e</math>)
  </th>
</tr>
<tr>
  <td align="center" colspan="3">
<table border="0" cellpadding="3">
<table border="0" cellpadding="3">
<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
<math>
~\theta_1 = \frac{\sin\xi_e}{\xi_e}  
\frac{R_\mathrm{SWS}}{R_\mathrm{rf}}  
</math>
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
&nbsp; &nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; &nbsp;
<math>~=~</math>
   </td>
   </td>
 
   <td align="left">
   <td align="right">
<math>
<math>
~\frac{d\theta_1}{d\xi} \biggr|_{\xi_e} = \frac{\cos\xi_e}{\xi_e} - \frac{\sin\xi_e}{\xi_e^2}
~\biggl[ \frac{3^{(n+1)}}{2^{(n+3)}} \biggl( \frac{5}{n+1}  \biggr)^n \biggr]^{1/(n-3)} \biggl( \frac{\pi}{n} \biggr)^{1/2} p_a^{(1-n)/[2(n+1)]} \, .
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
</table>
  </td>
</div>
</tr>


<tr>
</td></tr>
  <td align="center" rowspan="1">
</table>
[http://adsabs.harvard.edu/abs/1970MNRAS.151...81H Horedt (1970)]
</div>
<br>for<br>
 
fixed <math>~(M,K_n)</math>
===Chieze's Presentation===
From equations (8), (10), and (68) in Chapter IV of [<b>[[User:Tohline/Appendix/References#C67|<font color="red">C67</font>]]</b>], we can immediately formulate the following expressions for, respectively, <math>~P_e(\tilde\xi), R_\mathrm{eq}(\tilde\xi)</math>, and <math>~M_\mathrm{tot}(\tilde\xi)</math>:
 
<div align="center">
<table border="0" cellpadding="5" align="center">
 
<tr>
  <td align="right">
<math>~\frac{P_e}{P_\mathrm{Ch}}</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
<math>
<math>~=</math>
~r_a = \frac{R_\mathrm{eq}}{R_\mathrm{Horedt}} = \xi_e
</math>
   </td>
   </td>
   <td align="center">
   <td align="left">
<math>
<math>~ {\tilde\theta}^{n+1} \, ,</math>
~p_a = \frac{P_e}{P_\mathrm{Horedt}} = \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2
</math>
   </td>
   </td>
</tr>
</tr>


<tr>
<tr>
   <td align="center" rowspan="1">
   <td align="right">
[http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth (1981)]
<math>~\frac{R_\mathrm{eq}}{R_\mathrm{Ch}}</math>
<br>for<br>
fixed <math>~(M,K_n)</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
<math>
<math>~=</math>
\frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl( \frac{2^3}{3^2 \cdot 5} \biggr)^{1/2} \xi_e
   </td>
</math>
   <td align="left">
   </td>
<math>~\biggl[ \frac{n+1}{4\pi} \biggr]^{1 / 2} \tilde\xi</math>
   <td align="center">
<math>
\frac{P_e}{P_\mathrm{rf}} = \biggl( \frac{3^4 \cdot 5^3}{2^7} \biggr) \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2
</math>
   </td>
   </td>
</tr>
</tr>


<tr>
<tr>
   <td align="center" rowspan="1">
   <td align="right">
[http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler (1983)]
<math>~\frac{M_\mathrm{tot}}{M_\mathrm{Ch}}</math>
<br>for<br>
fixed <math>~(P_e,K_n)</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
<math>
<math>~=</math>
\frac{R_\mathrm{eq}}{R_\mathrm{SWS}} = (4\pi)^{-1/2} \xi_e
</math>
   </td>
   </td>
   <td align="center">
   <td align="left">
<math>
<math>~
\frac{M}{M_\mathrm{SWS}} = (4\pi)^{-1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr]
\biggl[ \frac{(n+1)^3}{4\pi} \biggr]^{1 / 2}(- {\tilde\xi}^2 {\tilde\theta}^') \, ,
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
</div>
where,
<div align="center">
<table border="0" cellpadding="5" align="center">


<tr>
<tr>
   <td align="left" colspan="3">
   <td align="right">
NOTE:  None of the analytic expressions for the dimensionless radius, pressure, or mass presented in this table explicitly appear in the referenced articles by Horedt, by Whitworth, or by Stahler but, as is discussed fully above, they are straightforwardly derivable from the more general relations that appear in these papers. 
<math>~P_\mathrm{Ch}</math>
  </td>
  <td align="center">
<math>~\equiv</math>
  </td>
  <td align="left">
<math>~K\rho_c^{(n+1)/n} \, ,</math>
   </td>
   </td>
</tr>
</tr>
</table>
</div>
</span>


=={{User:Tohline/Math/MP_PolytropicIndex}} = 5 Polytrope==
Drawing from the [[User:Tohline/SSC/Structure/Polytropes|earlier discussion of isolated polytropes]], we will reference various radial locations within a spherical {{User:Tohline/Math/MP_PolytropicIndex}} = 5 polytrope by the dimensionless radius,
<div align="center">
<math>
\xi \equiv \frac{r}{a_\mathrm{n=5}} ,
</math>
</div>
where,
<div align="center">
<math>
a_{n=5} =  \biggr[ \frac{(n+1)K}{4\pi G} \rho_c^{(1/n - 1)} \biggr]^{1/2}_{n=5} =
\biggr[ \frac{3K}{2\pi G} \biggr]^{1/2}  \rho_c^{-2/5}  \, .
</math>
</div>
The solution to the Lane-Emden equation for <math>~n = 5</math> is,
<div align="center">
<table border="0" cellpadding="3">
<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
<math>~R_\mathrm{Ch}</math>
~\theta_5
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
<math>~=~</math>
<math>~\equiv</math>
   </td>
   </td>
   <td align="left">
   <td align="left">
<math>
<math>~\biggl[\biggl(\frac{K}{G}\biggr) \rho_c^{1/n-1}\biggr]^{1 / 2} \, ,</math>
\biggl(1+\frac{\xi^2}{3} \biggr)^{-1/2} \, ,
</math>
   </td>
   </td>
</tr>
</tr>
</table>
 
</div>
hence,
<div align="center">
<table border="0" cellpadding="3">
<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
<math>~M_\mathrm{Ch}</math>
\frac{d\theta_5}{d\xi}
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
<math>~=~</math>
<math>~\equiv</math>
   </td>
   </td>
   <td align="left">
   <td align="left">
<math>
<math>~\biggl[\biggl(\frac{K}{G}\biggr)^3 \rho_c^{(3-n)/n}\biggr]^{1 / 2} \, .</math>
- \frac{\xi}{3}\biggl(1+\frac{\xi^2}{3} \biggr)^{-3/2} \, .
</math>
   </td>
   </td>
</tr>
</tr>
Line 1,185: Line 1,119:
</div>
</div>


<font color="darkblue">
In this case, the expressions for the physical variable normalizations have been defined in terms of  &#8212; in addition to <math>~G</math> and/or <math>~K</math> &#8212; the equilibrium configuration's central density, <math>~\rho_c</math>, instead of in terms of <math>~M_\mathrm{tot}</math> or <math>~P_e</math>.  These are precisely the expressions for, respectively, <math>~P_s(\xi_s)</math>, <math>~R_s(\xi_s)</math>, and <math>~M_s(\xi_s)</math> that appear in the appendix of [http://adsabs.harvard.edu/abs/1987A%26A...171..225C J. P. Chieze (1987, A&amp;A, 171, 225-232)] &#8212; see, respectively, his equations (A7), (A5), and (A6).  [Note that, for the polytropic systems of interest to us, here &#8212; that is, systems having <math>~0 \le n < \infty</math> &#8212; Chieze's parameter <math>~\epsilon \equiv \sgn(n+1) = 1</math>.]
===Review===
 
</font>
==Polytropic Configurations with n = 1==


Again, from the [[User:Tohline/SSC/Structure/Polytropes|earlier discussion]], we can describe the properties of an isolated, spherical {{User:Tohline/Math/MP_PolytropicIndex}} = 5 polytrope as follows:
Drawing from the [[User:Tohline/SSC/Structure/Polytropes|earlier discussion of isolated polytropes]], we will reference various radial locations within the spherical configuration by the dimensionless radius,
* <font color="red">Mass</font>: 
: In terms of the central density, <math>\rho_c</math>, and {{User:Tohline/Math/MP_PolytropicConstant}}, the total mass is,
<div align="center">
<div align="center">
<math>M = \biggr[ \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr]^{1/2} \rho_c^{-1/5} </math> ;
<math>
\xi \equiv \frac{r}{a_\mathrm{n=1}} ,
</math>
</div>
</div>
: and, expressed as a function of <math>M</math>, the mass that lies interior to the dimensionless radius <math>\xi</math> is,
where,
<div align="center">
<div align="center">
<math>
<math>
\frac{M_\xi}{M} = \xi^3 (3 + \xi^2)^{-3/2} \, .
a_\mathrm{n=1} \equiv \biggl[\frac{1}{4\pi G}~ \biggl( \frac{H_c}{\rho_c} \biggr)_{n=1}\biggr]^{1/2} = \biggl[\frac{K}{2\pi G} \biggr]^{1/2} \, .
</math>  
</math>
</div>
</div>
: Hence,
The solution to the Lane-Emden equation for <math>~n = 1</math> is,
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
<math>
M_\xi = \biggr[ \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr]^{1/2} \rho_c^{-1/5} \biggl[ \xi^3 (3 + \xi^2)^{-3/2} \biggr] \, .
~\theta_1
</math>  
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\frac{\sin\xi}{\xi} \, ,
</math>
  </td>
</tr>
</table>
</div>
</div>
 
hence,
* <font color="red">Pressure</font>:
: The central pressure of the configuration is,
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
<math>
P_c = \biggr[ \frac{\pi M^2 G^3}{2\cdot 3^4} \biggr]^{1/3}  \rho_c^{4/3} = \biggr[ \frac{\pi G^3}{2\cdot 3^4}
\frac{d\theta_1}{d\xi}
\biggr( \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr) \rho_c^{-2/5}\biggr]^{1/3}  \rho_c^{4/3} = K\rho_c^{6/5}
</math>
</math> ;
   </td>
</div>
   <td align="center">
 
<math>~=~</math>
: and, expressed in terms of the central pressure <math>P_c</math>, the variation with radius of the pressure is,
  </td>
<div align="center">
<math>P_\xi= P_c \biggl[  1 + \frac{1}{3}\xi^2 \biggr]^{-3}</math> .
</div>
: Hence,
<div align="center">
<math>
P_\xi = K \rho_c^{6/5}  \biggl[  1 + \frac{1}{3}\xi^2 \biggr]^{-3} =
3^3K \rho_c^{6/5}  [  3 + \xi^2 ]^{-3}  
</math> .
</div>
 
===Extension to Bounded Sphere===
Eliminating <math>\rho_c</math> between the last expression for <math>M_\xi</math> and the last expression for <math>P_\xi</math> gives,
<div align="center">
<table border="0" cellpadding="5">
<tr>
   <td align="right"><math>~P_\xi</math></td>
   <td align="center"><math>=</math></td>
   <td align="left">
   <td align="left">
<math>
<math>
3^3K  [  3 + \xi^2 ]^{-3} \biggr[ \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr]^{3}  M_\xi^{-6} \biggl[ \xi^3 (3 + \xi^2)^{-3/2} \biggr]^6
\frac{\cos\xi}{\xi} - \frac{\sin\xi}{\xi^2} \, .
</math>
</math>
   </td>
   </td>
</tr>
</tr>
</table>
</div>


<tr>
<font color="darkblue">
  <td align="right">&nbsp;</td>
===Review===
  <td align="center"><math>=</math></td>
</font>
  <td align="left">
 
<math>
Again, from the [[User:Tohline/SSC/Structure/Polytropes|earlier discussion]], we can describe the properties of an isolated, spherical {{User:Tohline/Math/MP_PolytropicIndex}} = 1 polytrope as follows:
\biggl( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 M_\xi^{6} G^9} \biggr)  \frac{\xi^{18}}{(3 + \xi^2)^{12}} \, .
* <font color="red">Mass</font>: 
</math>
: In terms of the central density, <math>\rho_c</math>, and {{User:Tohline/Math/MP_PolytropicConstant}}, the total mass is,
  </td>
<div align="center">
</tr>
<math>M = \frac{4}{\pi} \rho_c (\pi a_{n=1})^3 = 4\pi^2 \rho_c \biggl[\frac{K}{2\pi G} \biggr]^{3/2} = \rho_c \biggl[\frac{2\pi K^3}{G^3} \biggr]^{1/2}</math> ;
</table>
</div>
</div>
Now, if we rip off an outer layer of the star down to some dimensionless radius <math>\xi_e < \infty</math>, the interior of the configuration that remains &#8212; containing mass <math>M_{\xi_e}</math> &#8212; should remain in equilibrium if we impose the appropriate amount of externally applied pressure <math>P_e = P_{\xi_e} </math> at that radius.  (This will work only for spherically symmetric configurations, as the gravitation acceleration at any location only depends on the mass contained inside that radius.)  If we rescale our solution such that the mass enclosed within <math>\xi_e</math> is the original total mass <math>M</math>, then the pressure that must be imposed by the external medium in which the configuration is embedded is,
 
: and, expressed as a function of <math>M</math>, the mass that lies interior to the dimensionless radius <math>\xi</math> is,
<div align="center">
<div align="center">
<math>P_e= \biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 M^{6} G^9} \biggr) \frac{\xi_e^{18}}{(3 + \xi_e^2)^{12}} </math> .
<math>\frac{M_\xi}{M} = \frac{1}{\pi} \biggl[ \sin\xi - \xi\cos\xi \biggr] \, ,~~~~~~\mathrm{for}~\pi \ge \xi \ge 0 \, .</math>  
</div>
</div>
The associated equilibrium radius of this pressure-confined configuration is,
: Hence,
<div align="center">
<div align="center">
<math>
<math>M_\xi = \rho_c \biggl[\frac{2K^3}{\pi G^3} \biggr]^{1/2} \biggl[ \sin\xi - \xi\cos\xi \biggr] \, .</math>  
R_\mathrm{eq} = \xi_e a_\mathrm{n=5} = \biggl[ \frac{3K}{2\pi G} \biggr]^{1/2} \rho_c^{-2/5} \xi_e =
\biggl[ \frac{\pi M^4 G^5}{2^3 \cdot 3^7 K^5} \biggr]^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5} \, .
</math>
</div>
</div>


====Overlap with Whitworth's Presentation====
* <font color="red">Pressure</font>:
The curve labeled <math>~n=5</math> in the top two panels of Figure 1 shows how <math>R_\mathrm{eq}</math> varies with the applied external pressure <math>P_e</math>; as shown, the curve has two segments &#8212; configurations that are stable (blue diamonds) and configurations that are unstable (red squares).  Following the lead of [http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth] (1981, MNRAS, 195, 967) &#8212; for clarification, read the [[User:Tohline/SSC/Structure/PolytropesASIDE1|accompanying ASIDE]] &#8212; these two quantities have been respectively normalized (or, "referenced") to,
: The central pressure of the configuration is,
<div align="center">
<math>P_c = \biggl[ \frac{G^3}{2\pi} \rho_c^4 M^2 \biggr]^{1/3} = \biggl[ \frac{G^3}{2\pi} \rho_c^6 \biggl(\frac{2\pi K^3}{G^3} \biggr) \biggr]^{1/3} = K\rho_c^2</math> ;
</div>
 
: and, expressed in terms of the central pressure <math>P_c</math>, the variation with radius of the pressure is,
<div align="center">
<div align="center">
<math>
<math>P_\xi= P_c \biggl[ \frac{\sin\xi}{\xi} \biggr]^2</math> .
R_\mathrm{rf}\biggr|_\mathrm{n=5} \equiv \frac{2^6}{3^3} \biggl( \frac{\pi}{5^5} \biggr)^{1/2} \biggl[ \frac{G^5 M^4}{K^5} \biggr]^{1/2} ~~~\Rightarrow ~~~ \frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl( \frac{5^5}{2^{15}\cdot 3} \biggr)^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5} \, ;
</math>
</div>
</div>
and,
: Hence,
<div align="center">
<div align="center">
<math>
<math>P_\xi= K\rho_c^2 \biggl[ \frac{\sin\xi}{\xi} \biggr]^2</math> .
P_\mathrm{rf}\biggr|_\mathrm{n=5} \equiv \frac{3^{12} 5^9}{2^{26} \pi^3} \biggl( \frac{K^{10}}{G^9 M^6} \biggr) ~~~\Rightarrow ~~~ \frac{P_e}{P_\mathrm{rf}} = \biggl( \frac{2^{29}\cdot 3^{3} }{5^9}  \biggr) \frac{\xi_e^{18}}{(3 + \xi_e^2)^{12}}  \, .
</math>
</div>
</div>
We see that this <math>~n=5</math> model sequence bends back on itself.  That is to say, for this polytropic index there is an externally applied pressure above which no equilibrium configuration exists.  This limiting pressure arises along the curve where,  
 
===Extension to Bounded Sphere===
Eliminating <math>\rho_c</math> between the last expression for <math>M_\xi</math> and the last expression for <math>P_\xi</math> gives,
<div align="center">
<div align="center">
<math>\frac{dP_e}{dR_\mathrm{eq}} = \biggl( \frac{dP_e}{d\xi_e} \biggr) \biggl( \frac{dR_\mathrm{eq}}{d\xi_e} \biggr)^{-1} = 0 \, .</math>
<math>P_\xi= \biggl[\frac{\pi}{2} \cdot \frac{G^3 M_\xi^2}{K^2} \biggr] \biggl[ \frac{\sin\xi}{\xi(\sin\xi - \xi \cos\xi )} \biggr]^2</math> .
</div>
</div>
Evaluation of this expression shows that the limiting pressure occurs precisely at <math>\xi_e = 3</math>, that is,
Now, if we rip off an outer layer of the star down to some dimensionless radius <math>\xi_e < \pi</math>, the interior of the configuration that remains &#8212; containing mass <math>M_{\xi_e}</math> &#8212; should remain in equilibrium if we impose the appropriate amount of externally applied pressure <math>P_e = P_{\xi_e} </math> at that radius.  (This will work only for spherically symmetric configurations, as the gravitation acceleration at any location only depends on the mass contained inside that radius.)  If we rescale our solution such that the mass enclosed within <math>\xi_e</math> is the original total mass <math>M</math>, then the pressure that must be imposed by the external medium in which the configuration is embedded is,
<div align="center">
<div align="center">
<math>
<math>P_e= \biggl[\frac{\pi}{2} \cdot \frac{G^3 M^2}{K^2} \biggr] \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2</math> .
\biggl( \frac{P_e}{P_\mathrm{rf}} \biggr)_\mathrm{max} = \biggl( \frac{2^{29}\cdot 3^{3} }{5^9} \biggr) \frac{3^{18}}{12^{12}} = \frac{2^5 \cdot 3^9}{5^9} \, ,
</math>
</div>
</div>
and the radius of this limiting configuration is,
The associated equilibrium radius of this pressure-confined configuration is,
<div align="center">
<div align="center">
<math>
<math>
\biggl( \frac{R_\mathrm{eq}}{R_\mathrm{rf}} \biggr) = \biggl( \frac{5^5}{2^{15}\cdot 3} \biggr)^{1/2} \frac{12^3}{3^5} = \biggl( \frac{5^5}{2^3 \cdot 3^5} \biggr)^{1/2} \, .
R_\mathrm{eq} = \xi_e a_\mathrm{n=1} = \biggl[ \frac{K}{2\pi G} \biggr]^{1/2} \xi_e
</math>
</math>
</div>
</div>
On the log-log plot displayed in the top-right panel of Figure 1, the location of this special point is <math>[ \log(P_e/P_\mathrm{rf}) , \log(R_\mathrm{eq}/R_\mathrm{rf}) ] \approx [ -0.49149, +0.10308 ] \, .</math>


We note as well that a conversion from Whitworth's normalizations to the normalizations adopted by Horedt produce the following coordinates for the limiting model configuration:
====Overlap with Whitworth's Presentation====
The solid green curve in the two top panels of Figure 1 shows how <math>R_\mathrm{eq}</math> varies with the applied external pressure <math>P_e</math> for this pressure-bounded <math>~n=1</math> model sequence.  In the top-right panel, following the lead of [http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth] (1981, MNRAS, 195, 967) &#8212; for clarification, read the [[User:Tohline/SSC/Structure/PolytropesASIDE1|accompanying ASIDE]] &#8212; these two quantities have been respectively normalized (or, "referenced") to,
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
<math>
~p_a|_\mathrm{max}
R_\mathrm{rf}\biggr|_\mathrm{n=1} \equiv \biggl( \frac{3^2 \cdot 5}{2^4 \pi} \biggr)^{1/2} \biggl(\frac{K}{G}\biggr)^{1/2} ~~~\Rightarrow ~~~ \frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl( \frac{2^3}{3^2 \cdot 5} \biggr)^{1/2} \xi_e \, ;
</math>
</math>
  </td>
</div>
  <td align="center">
and,
<math>~=~</math>
<div align="center">
  </td>
  <td align="left">
<math>
<math>
~\frac{3^{12}}{2^{24}} \, ,
P_\mathrm{rf}\biggr|_\mathrm{n=1} \equiv \frac{2^6 \pi}{3^4 \cdot 5^3}  \biggl(\frac{G^3 M^2}{K^2}\biggr) ~~~\Rightarrow ~~~ \frac{P_e}{P_\mathrm{rf}} = \biggl( \frac{3^4 \cdot 5^3}{2^7} \biggr) \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2 \, .
</math>
</math>
  </td>
</tr>
</table>
</div>
and, at this bounding pressure, the model has an equilibrium radius,
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
~r_a
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\frac{2^6}{3^3} \, .
</math>
  </td>
</tr>
</table>
</div>
</div>
Note that this pair of mathematical expressions has been recorded to the immediate right of Whitworth's name in our [[User:Tohline/SSC/Structure/PolytropesEmbedded#n1Summary|<math>~n=1</math> summary table]].
In the top-left panel of Figure 1, the solid green curve shows the identical sequence, but plotted as <math>~\log(p_a)</math> versus <math>~log(r_a)</math>, for easier comparison with Horedt's work.  The pair of mathematical expressions defining <math>~r_a(\xi_e)</math> and <math>~p_a(\xi_e)</math> has been recorded to the immediate right of Horedt's name in the same [[User:Tohline/SSC/Structure/PolytropesEmbedded#n1Summary|summary table]].
<span id="WhitworthFig1b">
<div align="center">
<table border="2" cellpadding="8" width="85%">
<tr>
  <td align="center" colspan="2">
'''Figure 1:''' <font color="darkblue">
Equilibrium R-P Diagram &#8212; Referred to by [http://adsabs.harvard.edu/abs/1981PASJ...33..273K Kimura (1981)] as an "M<sub>1</sub> Sequence"
</font>
  </td>
</tr>
<tr>
  <td align="left" colspan="2">
All of the plots shown in this figure illustrate how the equilibrium radius of a pressure-bounded polytrope varies with the applied external pressure.  In the right-hand column, the log-log plots display a normalized <math>~P_e</math> along the horizontal axis and a normalized <math>~R_\mathrm{eq}</math> along the horizontal axis; in the left-hand column, these axes are flipped, and a different normalization is used.  One primary intent of all the diagrams is to show that, for polytropic sequences having <math>~n > 3</math> (or, equivalently, sequences having <math>\gamma_g \equiv 1 + 1/n < 4/3),</math> no equilibrium models exist above some limiting external pressure.
  </td>
</tr>
<tr>
  <td align="center" bgcolor="white">
[[File:HoredtPlot2.png|250px|center|To be compared with Horedt (1970)]]
  </td>
  <td align="center" bgcolor="white">
[[File:WhitworthPlot2.png|250px|center|To be compared with Whitworth (1981)]]
  </td>
</tr>
<tr>
  <td align="center" bgcolor="white">
[[File:Horedt_PRdiagram0.png|250px|center|Horedt (1970) Figure 1]]
<!-- [[Image:AAAwaiting01.png|250px|center|Horedt (1970) Figure 1]] -->
  </td>
  <td align="center" bgcolor="white">
[[File:WhitworthFig1bCopy.jpg|300px|center|Whitworth (1981) Figure 1b]]
<!--[[Image:AAAwaiting01.png|300px|center|Whitworth (1981) Figure 1b]] -->
  </td>
</tr>
<tr>
  <td align="center" bgcolor="white">
[[File:Horedt_EmbeddedPolytrope.png|300px|center|Horedt (1970) Title Page]]
<!--[[Image:AAAwaiting01.png|300px|center|Horedt (1970) Title Page]] -->
  </td>
  <td align="center" bgcolor="white">
[[File:Whitworth1981TitlePage0.png|200px|center|Whitworth (1981) Title Page]]
<!--[[Image:AAAwaiting01.png|200px|center|Whitworth (1981) Title Page]] -->
  </td>
</tr>
<tr>
  <td align="left" colspan="2">
''Bottom Left'' [reproduction of Figure 1 from [http://adsabs.harvard.edu/abs/1970MNRAS.151...81H Horedt (1970)]]:  All three displayed sequences &#8212; <math>~n=4</math> (<math>~\gamma_g = 1.25</math>), <math>~n=5</math> (<math>~\gamma_g = 1.20</math>), and <math>~n=\infty</math> (<math>~\gamma_g = 1</math>, hence, isothermal) &#8212; exhibit an upper limit for the bounding pressure.  Each sequence displays two segments &#8212; a solid segment and a dashed segment &#8212; indicating that, below the maximum allowed value of <math>~P_e</math>, it is possible to construct two (or more) equilibrium configurations; models lying along the solid segment of each displayed curve are expected to be dynamically stable while models lying along the dashed segments are unstable.
''Bottom Right'' [reproduction of Figure 1b from [http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth (1981)]]:  Model sequences are shown for five different effective adiabatic indexes &#8212; <math>~\gamma_g = 1/3,~ 2/3,~ 1,~ 4/3,</math> and <math>~ 5/3</math> &#8212; corresponding, respectively, to polytropic indexes <math>~n = -2/3, -1/3, \infty, ~3/2, </math> and <math>~3</math>.  The three sequences having <math>~\gamma_g < 4/3</math> exhibit an upper limit for the bounding pressure.  Both the stable (solid) curve segment and the unstable (dashed) curve segment are drawn for the isothermal <math>~(\gamma_g = 1)</math> sequence, which is also displayed (as the <math>~n=\infty</math> sequence) in Horedt's diagram.
''Top'':  Plots that we have generated for direct comparison with Horedt's diagram (''left'') and with Whitworth's diagram (''right'').  Both plots display only the two sequences that are analytically prescribable:  <math>~n=1</math> (<math>~\gamma_g = 2</math>) and <math>~n=5</math> (<math>~\gamma_g = 1.20</math>).  Along the <math>~n=1</math> (green) sequence, stable equilibrium models can be constructed for all values of <math>~P_e</math>.  Along the <math>~n=5</math> sequence, equilibrium models only exist for values of <math>~P_e</math> less than the critical value, <math>~P_\mathrm{max} = (2^5\cdot 3^9/5^9) P_\mathrm{rf} = (3^{12}/2^{24}) P_\mathrm{Horedt}</math>; below this critical pressure, the sequence has two branches denoted by blue diamonds (stable models) and red squares (unstable models).
  </td>
</tr>
</table>
</div>
</span>
====Overlap with Stahler's Presentation====
We can invert the above expression for <math>~P_e(K,M)</math> to obtain the following expression for <math>~M(K,P_e)</math>:
<div align="center">
<math>~M= K \biggl[\frac{2}{\pi} \cdot \frac{P_e}{G^3} \biggr]^{1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr]</math> .
</div>
If, following Stahler's lead, we normalize this expression by <math>~M_\mathrm{SWS}</math> (evaluated for <math>~n=1</math>) and we normalize the above expression for <math>~R_\mathrm{eq}</math> by <math>~R_\mathrm{SWS}</math> (evaluated for <math>~n=1</math>), we obtain,
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
\frac{M}{M_\mathrm{SWS}}
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
K \biggl[\frac{2}{\pi} \cdot \frac{P_e}{G^3} \biggr]^{1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr]
\biggl[ \biggl( \frac{G}{2} \biggr)^{3/2} K^{-1} P_\mathrm{ex}^{-1/2}  \biggr]
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
(4\pi)^{-1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr] \, ,
</math>
  </td>
</tr>
<tr>
  <td align="right">
<math>
\frac{R_\mathrm{eq}}{R_\mathrm{SWS}}
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\biggl[ \frac{K}{2\pi G} \biggr]^{1/2} \xi_e \biggl[ \frac{G}{2K} \biggr]^{1/2} =
(4\pi)^{-1/2} \xi_e \, .
</math>
  </td>
</tr>
</table>
</div>
<span id="Stahler1983Fig17">
<div align="center">
<table border="2" cellpadding="8">
<tr>
  <td align="center" colspan="2">
'''Figure 2:''' <font color="darkblue">Equilibrium Mass-Radius Diagram </font>
  </td>
</tr>
<tr>
  <td align="center" bgcolor="white">
[[File:Stahler1983TitlePage0.png|300px|center|Stahler (1983) Title Page]]
<!-- [[Image:AAAwaiting01.png|300px|center|Stahler (1983) Title Page]] -->
  </td>
  <td valign="top" width=350 rowspan="3">
''Top:'' A slightly edited reproduction of Figure 17 in association with Appendix B of  [http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler] (1983, ApJ, 268, 165).  Stahler's figure caption reads, in part, "Mass-radius relation for bounded polytropes (schematic).  Each curve is labeled by the appropriate value or range" of {{User:Tohline/Math/MP_PolytropicIndex}} &hellip; "As the cloud density increases from unity, all curves leave the origin with the same slope &hellip;" 
''Bottom:''  Curves depict the exact, analytically derived mass-radius relationship for truncated <math>~n = 1</math> (purple squares) and <math>~n = 5</math> (blue diamonds) polytropes that are embedded in an external medium of pressure <math>~P_e</math>; the relevant mathematical expressions are presented to the immediate right of Stahler's name in, respectively, our [[User:Tohline/SSC/Structure/PolytropesEmbedded#n1Summary|<math>~n=1</math> summary table]] and our [[User:Tohline/SSC/Structure/PolytropesEmbedded#n5Summary|<math>~n=5</math> summary table]].  As the dimensionless truncation radius, <math>~\xi_e</math>, increases steadily from zero, both curves exhibit very similar behavior up to <math>~M_n \equiv M/M_\mathrm{SWS} \approx 0.5</math>; thereafter the normalized mass and normalized radius continue to steadily increase along the <math>~n = 1</math> sequence, but the <math>~n = 5</math> sequence eventually bends back on itself, returning to the origin as <math>~\xi_e \rightarrow \infty</math>. 
''Comparison:'' The monotonic <math>P-R</math> behavior of the analytically derived solution for {{User:Tohline/Math/MP_PolytropicIndex}} = 1 <math>(\gamma_g = 2)</math>, shown above, is consistent with the behavior of the numerically derived solutions presented by Whitworth for slightly lower values of <math>\gamma_g</math> = 5/3 and 4/3.  The analytically derived solution for {{User:Tohline/Math/MP_PolytropicIndex}} = 5 <math>(\gamma_g = 6/5)</math> shows that, above some limiting pressure, no equilibrium configuration exists; this is consistent with the behavior of the numerically derived solutions presented by Whitworth for all values of <math>\gamma_g < 4/3 \, .</math>
</td>
</tr>
<tr>
  <td align="center" bgcolor="white">
[[File:Stahler_MRdiagram1.png|300px|center|Stahler (1983) Figure 17 (edited)]]
<!-- [[Image:AAAwaiting01.png|300px|center|Stahler (1983) Figure 17 (edited)]] -->
  </td>
</tr>
<tr>
  <td align="center" bgcolor="white">
[[File:Stahler1983Comparison.png|300px|center|To be compared with Stahler (1983)]]
  </td>
</tr>
</table>
</div>
</span>
===Tabular Summary (n=1) ===
<span id="n1Summary">
<div align="center">
<table border="1" cellpadding="8" width="95%">
<tr>
  <th align="center" colspan="3">
Table 1: &nbsp;Properties of <math>~n=1</math> Polytropes Embedded in an External Medium of Pressure <math>~P_e</math>
<br>
(and, accordingly, truncated at radius <math>~\xi_e</math>)
  </th>
</tr>
<tr>
  <td align="center" colspan="3">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
~\theta_1 = \frac{\sin\xi_e}{\xi_e}
</math>
  </td>
  <td align="center">
&nbsp; &nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; &nbsp;
  </td>
  <td align="right">
<math>
~\frac{d\theta_1}{d\xi} \biggr|_{\xi_e} = \frac{\cos\xi_e}{\xi_e} - \frac{\sin\xi_e}{\xi_e^2}
</math>
  </td>
</tr>
</table>
  </td>
</tr>
<tr>
  <td align="center" rowspan="1">
[http://adsabs.harvard.edu/abs/1970MNRAS.151...81H Horedt (1970)]
<br>for<br>
fixed <math>~(M,K_n)</math>
  </td>
  <td align="center">
<math>
~r_a = \frac{R_\mathrm{eq}}{R_\mathrm{Horedt}} = \xi_e
</math>
  </td>
  <td align="center">
<math>
~p_a = \frac{P_e}{P_\mathrm{Horedt}} = \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2
</math>
  </td>
</tr>
<tr>
  <td align="center" rowspan="1">
[http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth (1981)]
<br>for<br>
fixed <math>~(M,K_n)</math>
  </td>
  <td align="center">
<math>
\frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl( \frac{2^3}{3^2 \cdot 5} \biggr)^{1/2} \xi_e
</math>
  </td>
  <td align="center">
<math>
\frac{P_e}{P_\mathrm{rf}} = \biggl( \frac{3^4 \cdot 5^3}{2^7} \biggr) \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2
</math>
  </td>
</tr>
<tr>
  <td align="center" rowspan="1">
[http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler (1983)]
<br>for<br>
fixed <math>~(P_e,K_n)</math>
  </td>
  <td align="center">
<math>
\frac{R_\mathrm{eq}}{R_\mathrm{SWS}} = (4\pi)^{-1/2} \xi_e
</math>
  </td>
  <td align="center">
<math>
\frac{M}{M_\mathrm{SWS}} = (4\pi)^{-1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr]
</math>
  </td>
</tr>
<tr>
  <td align="left" colspan="3">
NOTE:  None of the analytic expressions for the dimensionless radius, pressure, or mass presented in this table explicitly appear in the referenced articles by Horedt, by Whitworth, or by Stahler but, as is discussed fully above, they are straightforwardly derivable from the more general relations that appear in these papers. 
  </td>
</tr>
</table>
</div>
</span>
==Polytropic Configurations with n = 5==
Drawing from the [[User:Tohline/SSC/Structure/Polytropes|earlier discussion of isolated polytropes]], we will reference various radial locations within a spherical {{User:Tohline/Math/MP_PolytropicIndex}} = 5 polytrope by the dimensionless radius,
<div align="center">
<math>
\xi \equiv \frac{r}{a_\mathrm{n=5}} ,
</math>
</div>
where,
<div align="center">
<math>
a_{n=5} =  \biggr[ \frac{(n+1)K}{4\pi G} \rho_c^{(1/n - 1)} \biggr]^{1/2}_{n=5} =
\biggr[ \frac{3K}{2\pi G} \biggr]^{1/2}  \rho_c^{-2/5}  \, .
</math>
</div>
The solution to the Lane-Emden equation for <math>~n = 5</math> is,
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
~\theta_5
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\biggl(1+\frac{\xi^2}{3} \biggr)^{-1/2} \, ,
</math>
  </td>
</tr>
</table>
</div>
hence,
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
\frac{d\theta_5}{d\xi}
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
- \frac{\xi}{3}\biggl(1+\frac{\xi^2}{3} \biggr)^{-3/2} \, .
</math>
  </td>
</tr>
</table>
</div>
<font color="darkblue">
===Review===
</font>
Again, from the [[User:Tohline/SSC/Structure/Polytropes|earlier discussion]], we can describe the properties of an isolated, spherical {{User:Tohline/Math/MP_PolytropicIndex}} = 5 polytrope as follows:
* <font color="red">Mass</font>: 
: In terms of the central density, <math>\rho_c</math>, and {{User:Tohline/Math/MP_PolytropicConstant}}, the total mass is,
<div align="center">
<math>M = \biggr[ \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr]^{1/2}  \rho_c^{-1/5}  </math> ;
</div>
: and, expressed as a function of <math>M</math>, the mass that lies interior to the dimensionless radius <math>\xi</math> is,
<div align="center">
<math>
\frac{M_\xi}{M} = \xi^3 (3 + \xi^2)^{-3/2}  \, .
</math>
</div>
: Hence,
<div align="center">
<math>
M_\xi = \biggr[ \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr]^{1/2}  \rho_c^{-1/5} \biggl[ \xi^3 (3 + \xi^2)^{-3/2} \biggr] \, .
</math>
</div>
* <font color="red">Pressure</font>:
: The central pressure of the configuration is,
<div align="center">
<math>
P_c = \biggr[ \frac{\pi M^2 G^3}{2\cdot 3^4} \biggr]^{1/3}  \rho_c^{4/3} = \biggr[ \frac{\pi G^3}{2\cdot 3^4}
\biggr( \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr) \rho_c^{-2/5}\biggr]^{1/3}  \rho_c^{4/3} = K\rho_c^{6/5}
</math> ;
</div>
: and, expressed in terms of the central pressure <math>P_c</math>, the variation with radius of the pressure is,
<div align="center">
<math>P_\xi= P_c \biggl[  1 + \frac{1}{3}\xi^2 \biggr]^{-3}</math> .
</div>
: Hence,
<div align="center">
<math>
P_\xi = K \rho_c^{6/5}  \biggl[  1 + \frac{1}{3}\xi^2 \biggr]^{-3} =
3^3K \rho_c^{6/5}  [  3 + \xi^2 ]^{-3}
</math> .
</div>
===Extension to Bounded Sphere===
Eliminating <math>\rho_c</math> between the last expression for <math>M_\xi</math> and the last expression for <math>P_\xi</math> gives,
<div align="center">
<table border="0" cellpadding="5">
<tr>
  <td align="right"><math>~P_\xi</math></td>
  <td align="center"><math>=</math></td>
  <td align="left">
<math>
3^3K  [  3 + \xi^2 ]^{-3} \biggr[ \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr]^{3}  M_\xi^{-6} \biggl[ \xi^3 (3 + \xi^2)^{-3/2} \biggr]^6
</math>
  </td>
</tr>
<tr>
  <td align="right">&nbsp;</td>
  <td align="center"><math>=</math></td>
  <td align="left">
<math>
\biggl( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 M_\xi^{6} G^9} \biggr)  \frac{\xi^{18}}{(3 + \xi^2)^{12}} \, .
</math>
  </td>
</tr>
</table>
</div>
Now, if we rip off an outer layer of the star down to some dimensionless radius <math>\xi_e < \infty</math>, the interior of the configuration that remains &#8212; containing mass <math>M_{\xi_e}</math> &#8212; should remain in equilibrium if we impose the appropriate amount of externally applied pressure <math>P_e = P_{\xi_e} </math> at that radius.  (This will work only for spherically symmetric configurations, as the gravitation acceleration at any location only depends on the mass contained inside that radius.)  If we rescale our solution such that the mass enclosed within <math>\xi_e</math> is the original total mass <math>M</math>, then the pressure that must be imposed by the external medium in which the configuration is embedded is,
<div align="center">
<math>P_e= \biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 M^{6} G^9} \biggr) \frac{\xi_e^{18}}{(3 + \xi_e^2)^{12}} </math> .
</div>
The associated equilibrium radius of this pressure-confined configuration is,
<div align="center">
<math>
R_\mathrm{eq} = \xi_e a_\mathrm{n=5} = \biggl[ \frac{3K}{2\pi G} \biggr]^{1/2} \rho_c^{-2/5} \xi_e =
\biggl[ \frac{\pi M^4 G^5}{2^3 \cdot 3^7 K^5} \biggr]^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5} \, .
</math>
</div>
====Overlap with Whitworth's Presentation====
The curve labeled <math>~n=5</math> in the top two panels of Figure 1 shows how <math>R_\mathrm{eq}</math> varies with the applied external pressure <math>P_e</math>; as shown, the curve has two segments &#8212; configurations that are stable (blue diamonds) and configurations that are unstable (red squares).  Following the lead of [http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth] (1981, MNRAS, 195, 967) &#8212; for clarification, read the [[User:Tohline/SSC/Structure/PolytropesASIDE1|accompanying ASIDE]] &#8212; these two quantities have been respectively normalized (or, "referenced") to,
<div align="center">
<math>
R_\mathrm{rf}\biggr|_\mathrm{n=5} \equiv \frac{2^6}{3^3} \biggl( \frac{\pi}{5^5} \biggr)^{1/2} \biggl[ \frac{G^5 M^4}{K^5} \biggr]^{1/2} ~~~\Rightarrow ~~~ \frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl( \frac{5^5}{2^{15}\cdot 3} \biggr)^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5} \, ;
</math>
</div>
and,
<div align="center">
<math>
P_\mathrm{rf}\biggr|_\mathrm{n=5} \equiv \frac{3^{12} 5^9}{2^{26} \pi^3} \biggl( \frac{K^{10}}{G^9 M^6} \biggr) ~~~\Rightarrow ~~~ \frac{P_e}{P_\mathrm{rf}} = \biggl( \frac{2^{29}\cdot 3^{3} }{5^9}  \biggr) \frac{\xi_e^{18}}{(3 + \xi_e^2)^{12}}  \, .
</math>
</div>
We see that this <math>~n=5</math> model sequence bends back on itself.  That is to say, for this polytropic index there is an externally applied pressure above which no equilibrium configuration exists.  This limiting pressure arises along the curve where,
<div align="center">
<math>\frac{dP_e}{dR_\mathrm{eq}} = \biggl( \frac{dP_e}{d\xi_e} \biggr) \biggl( \frac{dR_\mathrm{eq}}{d\xi_e} \biggr)^{-1} = 0 \, .</math>
</div>
Evaluation of this expression shows that the limiting pressure occurs precisely at <math>\xi_e = 3</math>,  that is,
<div align="center">
<math>
\biggl( \frac{P_e}{P_\mathrm{rf}} \biggr)_\mathrm{max} = \biggl( \frac{2^{29}\cdot 3^{3} }{5^9}  \biggr) \frac{3^{18}}{12^{12}} = \frac{2^5 \cdot 3^9}{5^9} \, ,
</math>
</div>
and the radius of this limiting configuration is,
<div align="center">
<math>
\biggl( \frac{R_\mathrm{eq}}{R_\mathrm{rf}} \biggr) = \biggl( \frac{5^5}{2^{15}\cdot 3} \biggr)^{1/2} \frac{12^3}{3^5} = \biggl( \frac{5^5}{2^3 \cdot 3^5} \biggr)^{1/2} \, .
</math>
</div>
On the log-log plot displayed in the top-right panel of Figure 1, the location of this special point is <math>[ \log(P_e/P_\mathrm{rf}) , \log(R_\mathrm{eq}/R_\mathrm{rf}) ] \approx [ -0.49149, +0.10308 ] \, .</math>
We note as well that a conversion from Whitworth's normalizations to the normalizations adopted by Horedt produce the following coordinates for the limiting model configuration:
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
~p_a|_\mathrm{max}
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
~\frac{3^{12}}{2^{24}} \, ,
</math>
  </td>
</tr>
</table>
</div>
and, at this bounding pressure, the model has an equilibrium radius,
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
~r_a
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\frac{2^6}{3^3} \, .
</math>
  </td>
</tr>
</table>
</div>
====Overlap with Stahler's Presentation====
We can invert the above expression for <math>~P_e(K,M)</math> to obtain the following expression for <math>~M(K,P_e)</math>:
<div align="center">
<math>M^{6}= \biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 P_e G^9} \biggr) \frac{\xi_e^{18}}{(3 + \xi_e^2)^{12}} </math> .
</div>
If, following Stahler's lead, we normalize this expression by <math>~M_\mathrm{SWS}</math> (evaluated for <math>~n=5</math>) and we normalize the above expression for <math>~R_\mathrm{eq}</math> by <math>~R_\mathrm{SWS}</math> (evaluated for <math>~n=5</math>), we obtain,
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
\frac{M}{M_\mathrm{SWS}}
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 P_e G^9} \biggr)^{1/6} \frac{\xi_e^{3}}{(3 + \xi_e^2)^{2}}
\biggl[ \biggl( \frac{2\cdot 3}{5G} \biggr)^{3/2} K^{5/3} P_\mathrm{ex}^{-1/6}  \biggr]^{-1}
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\biggr( \frac{3^{2} \cdot 5^3 }{4\pi } \biggr)^{1/2} \frac{\xi_e^{3}}{(3 + \xi_e^2)^{2}}  \, ,
</math>
  </td>
</tr>
<tr>
  <td align="right">
<math>
\frac{R_\mathrm{eq}}{R_\mathrm{SWS}}
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\biggl[ \frac{\pi M^4 G^5}{2^3 \cdot 3^7 K^5} \biggr]^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5}
\biggl[  \biggl( \frac{2\cdot 3}{5G} \biggr)^{1/2} K^{5/6} P_\mathrm{ex}^{-1/3}
\biggr]^{-1}
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\biggl[ \biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 P_e G^9} \biggr)^{1/3} \frac{\xi_e^{6}}{(3 + \xi_e^2)^{4}}  \biggr]
\biggl[ \frac{\pi G^5}{2^3 \cdot 3^7 K^5} \biggr]^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5}
\biggl( \frac{5G}{2\cdot 3} \biggr)^{1/2} \biggl[  K^{-5/6} P_\mathrm{ex}^{1/3} \biggr]
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\biggr( \frac{3^{2} \cdot 5}{2^2 \pi } \biggr)^{1/2}  \frac{\xi_e}{(3 + \xi_e^2)} \, .
</math>
  </td>
</tr>
</table>
</div>
This set of parametric relations that relate the mass of the truncated configuration to its radius via the parameter, <math>~\xi_e</math>, has been recorded to the immediate right of Stahler's name in our [[User:Tohline/SSC/Structure/PolytropesEmbedded#n5Summary|<math>~n=5</math> summary table]], below. 
Stahler points out (see his equation B13) that, for this particular pressure-bounded polytropic sequence, <math>~\xi_e</math> can be eliminated between the expressions to obtain the following direct algebraic relationship between <math>~M</math> and <math>~R_\mathrm{eq}</math>:
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
\biggl( \frac{M}{M_\mathrm{SWS}} \biggr)^2 - 5 \biggl( \frac{M}{M_\mathrm{SWS}} \biggr)\biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)
+ \frac{20\pi}{3} \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)^4
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
~0 \, .
</math>
  </td>
</tr>
</table>
</div>
Viewed as a quadratic equation in the mass, the roots of this expression give,
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
\frac{M}{M_\mathrm{SWS}}
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\frac{5}{2} \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr) \biggl\{ 1 \pm \biggl[ 1 - \frac{16\pi}{15}
\biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)^2 \biggr]^{1/2}  \biggr\} \, .
</math>
  </td>
</tr>
</table>
</div>
[<font color="red">CORRECTION:  Changed factor inside square root from <math>~16\pi/3</math> to <math>~16\pi/15</math> on 24 December 2014.</font>]  We have used this expression to generate the complete <math>~n=5</math> sequence shown here in the top panel of Figure 2 &#8212; the solid green segment of the curve shows the negative root and the solid red segment of the curve was generated using the positive root.
ASIDE:  In his Appendix B, [http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler (1983)] claims that the quadratic equation relating <math>~M</math> directly to <math>~R_\mathrm{eq}</math> (his equation B13) can be obtained by analytically integrating the first-order ordinary differential equation presented as his equation B10.  I don't think that this is possible without knowing ahead of time how <math>~M</math> relates to <math>~R_\mathrm{eq}</math> through the above-derived parametric relations in <math>~\xi_e</math>.
[<font color="red">29 September 2014</font> by J. E. Tohline] Now that (I think) I've finished deriving the properly defined [[User:Tohline/SSC/Virial/Polytropes#Nonrotating_Adiabatic_Configuration_Embedded_in_an_External_Medium|virial equilibrium condition for embedded polytropes]] and have reconciled that equilibrium expression with Horedt's corresponding specification of the equilibrium radius and surface-pressure, it's time to [[User:Tohline/SSC/Structure/StahlerMassRadius|revisit the concern]] that was expressed in this "ASIDE" regarding the mass-radius relationship for embedded, <math>~n=5</math> polytropes presented by Stahler.
===Tabular Summary (n=5) ===
<span id="n5Summary">
<div align="center">
<table border="1" cellpadding="8" width="95%">
<tr>
  <th align="center" colspan="3">
Table 2: &nbsp;Properties of <math>~n=5</math> Polytropes Embedded in an External Medium of Pressure <math>~P_e</math>
<br>
(and, accordingly, truncated at radius <math>~\xi_e</math>)
  </th>
</tr>
<tr>
  <td align="center" colspan="3">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
~\theta_5 = \biggl( 1 + \frac{\xi_e^2}{3} \biggr)^{-1/2}
</math>
  </td>
  <td align="center">
&nbsp; &nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; &nbsp;
  </td>
  <td align="right">
<math>
~\frac{d\theta_5}{d\xi} \biggr|_{\xi_e} = - \frac{\xi_e}{3} \biggl( 1 + \frac{\xi_e^2}{3} \biggr)^{-3/2}
</math>
  </td>
</tr>
</table>
  </td>
</tr>
<tr>
  <td align="center" rowspan="1">
[http://adsabs.harvard.edu/abs/1970MNRAS.151...81H Horedt (1970)]
<br>for<br>
fixed <math>~(M,K_n)</math>
  </td>
  <td align="center">
<math>
~r_a = \frac{R_\mathrm{eq}}{R_\mathrm{Horedt}} = \biggl\{ 3 \biggl[ \frac{(\xi_e^2/3)^5}{(1+\xi_e^2/3)^{6}} \biggr] \biggr\}^{-1/2}
</math>
  </td>
  <td align="center">
<math>
~p_a = \frac{P_e}{P_\mathrm{Horedt}} = 3^3 \biggl[ \frac{(\xi_e^2/3)^3}{(1+\xi_e^2/3)^{4}} \biggr]^3
</math>
  </td>
</tr>
<tr>
  <td align="center" rowspan="1">
[http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth (1981)]
<br>for<br>
fixed <math>~(M,K_n)</math>
  </td>
  <td align="center">
<math>
\frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl\{ \frac{2^{15}}{5^5} \biggl[ \frac{(\xi_e^2/3)^5}{(1+\xi_e^2/3)^{6}} \biggr] \biggr\}^{-1/2}
</math>
  </td>
  <td align="center">
<math>
\frac{P_e}{P_\mathrm{rf}} = \frac{2^{29}}{5^9} \biggl[ \frac{(\xi_e^2/3)^3}{(1+\xi_e^2/3)^{4}} \biggr]^3
</math>
  </td>
</tr>
<tr>
  <td align="center" rowspan="2">
[http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler (1983)]
<br>for<br>
fixed <math>~(P_e,K_n)</math>
  </td>
  <td align="center">
<math>
\frac{R_\mathrm{eq}}{R_\mathrm{SWS}} = \biggl\{ \frac{3\cdot 5}{2^2 \pi} \biggl[ \frac{\xi_e^2/3}{(1+\xi_e^2/3)^{2}} \biggr] \biggr\}^{1/2}
</math>
  </td>
  <td align="center">
<math>
\frac{M}{M_\mathrm{SWS}} = \biggl[  \biggl( \frac{3 \cdot 5^3}{2^2\pi} \biggr) \frac{(\xi_e^2/3)^3}{(1+\xi_e^2/3)^{4}} \biggr]^{1/2}
</math>
  </td>
</tr>
<tr>
  <td align="center" colspan="2">
<math>
\biggl( \frac{M}{M_\mathrm{SWS}} \biggr)^2 - 5 \biggl( \frac{M}{M_\mathrm{SWS}} \biggr)\biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)
+ \frac{2^2 \cdot 5 \pi}{3} \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)^4 = 0
</math>
  </td>
</tr>
<tr>
  <td align="left" colspan="3">
NOTE:  None of the analytic expressions for the dimensionless radius, pressure, or mass presented in this table explicitly appear in the referenced articles by Horedt, by Whitworth, or by Stahler but, as is discussed fully above, they are straightforwardly derivable from the more general relations that appear in these papers.  The final polynomial relating the dimensionless mass to the dimensionless radius ''does'' explicitly appear as equation (B13) in [http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler (1983)].
Additional discussion of Stahler's analytic mass-radius relation is presented in an  [[User:Tohline/SSC/Virial/PolytropesEmbedded/SecondEffortAgain#Plotting_Stahler.27s_Relation|accompanying chapter]].
  </td>
</tr>
</table>
</div>
</span>
==Additional, Numerically Constructed Polytropic Configurations==
As has been detailed in an [[User:Tohline/SSC/Structure/Polytropes#Polytropic_Spheres|accompanying chapter]], using numerical techniques we have solved the Lane-Emden equation, and thereby discerned the internal structural profiles, for polytropes having a wide variety of polytropic indexes.  The righthand panel of Figure 3 presents a diagram in which the mass-radius "sequences" corresponding to eight different polytropic indexes have been drawn.
<div align="center" id="DFBsequences">
<table border="1" cellpadding="8" align="center">
<tr>
  <th align="center" colspan="2"><br />[[File:DataFileButton02.png|right|75px|file = Dropbox/WorkFolder/Wiki edits/EmbeddedPolytropes/CombinedSequences.xlsx --- worksheet = EqSeqCombined2]]Figure 3: &nbsp; Mass-Radius Behavior of Various Polytropic Sequences</th>
</tr>
<tr>
  <td align="center">
[[File:Stahler_MRdiagram1.png|350px|center|Stahler (1983) Figure 17 (edited)]]
  </td>
  <td align="center">
[[File:DFBsequenceB.png|350px|Combined DFB Sequences]]
  </td>
</tr>
</table>
</div>
==Turning Points==
===Limiting Pressure Along M<sub>1</sub> Sequence===
As is illustrated in the figures presented above, when an equilibrium sequence is constructed for any bounded (pressure-truncated) configuration having <math>~n > 3</math>, the sequence exhibits multiple "turning points."  For example, when moving along the R-P sequence [[User:Tohline/SSC/Structure/PolytropesEmbedded#WhitworthFig1b|displayed in Figure 1]] for <math>~n=5</math> configurations, the external pressure monotonically climbs to a maximum value, <math>~P_\mathrm{max}</math>, then "turns around" and steadily decreases thereafter.  [http://adsabs.harvard.edu/abs/1970MNRAS.151...81H Horedt (1970)] and [http://adsabs.harvard.edu/abs/1981PASJ...33..299K Kimura (1981b)] separately derived an expression that pinpoints the location of the  <math>~P_\mathrm{max}</math> turning point along an R-P sequence &#8212; Kimura refers to this as an "M<sub>1</sub> sequence" because the configuration's mass is held fixed while the external pressure and the system's corresponding equilibrium radius is varied.  The turning point is located along the sequence at the point where,
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
\frac{d P_e}{d R_\mathrm{eq}} \biggr|_M
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
0\, ,
</math>
  </td>
</tr>
</table>
</div>
or, just as well, where,
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
\frac{d \ln P_e}{d\ln R_\mathrm{eq}} \biggr|_M
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
0\, .
</math>
  </td>
</tr>
</table>
</div>
In what follows, we examine the expressions derived by both authors in order to show that they are identical to one another as well as to re-express the result in a form that conforms to our own adopted notation.
====Horedt's Derivation====
Appreciating that Horedt's notation for the surface pressure of an equilibrium configuration &#8212; which equals the applied external pressure <math>~P_e</math> &#8212; is <math>~\tilde{p}</math>, and his notation for <math>~R_\mathrm{eq}</math> is <math>~\tilde{r}</math>, the requisite expression from Horedt's paper [see also equation (13) in [http://adsabs.harvard.edu/abs/1974A%26A....33..195V Viala &amp; Horedt (1974)]] is the one displayed in the following boxed image: 
<div align="center">
<table border="1" align="center" cellpadding="8">
<tr>
  <td align="center">
Excerpt from [http://adsabs.harvard.edu/abs/1970MNRAS.151...81H Horedt (1970)]
  </td>
</tr>
<tr><td align="left">
<table border="0" align="center">
<tr><td align="center">
[[File:HoredtEq00.png|450px|center|Viala &amp; Horedt (1974) Expressions]]
<!-- [[Image:AAAwaiting01.png|450px|center|Viala &amp; Horedt (1974) Expressions]]-->
</td></tr>
<tr><td align="left">
where,
</td></tr>
<tr><td align="center">
[[File:HoredtEq01.png|300px|center|Viala &amp; Horedt (1974) Expressions]]
<!-- [[Image:AAAwaiting01.png|300px|center|Viala &amp; Horedt (1974) Expressions]] -->
</td></tr>
</table>
</td></tr>
</table>
</div>
That is, from Horedt's work we have,
<div align="center">
<table border="0" align="center" cellpadding="5">
<tr>
  <td align="right">
<math>~\frac{dP_e}{dR_\mathrm{eq}}\biggr|_M ~~\rightarrow ~~ \frac{d\tilde{p}}{d \tilde{r}}</math>
  </td>
  <td align="center">
<math>~\sim</math>
  </td>
  <td align="left">
<math>~\frac{(3-n)(n+1)(\tilde\theta^')^2 + (2n+2)\tilde\theta^{n+1}}{(1-n)\tilde\xi f^' + (3-n)(n+1)(\tilde\theta^')^2} \, .</math>
  </td>
</tr>
</table>
</div>
Let's independently derive this relation, starting from Horedt's equilibrium expressions for <math>~\tilde{r}</math> and <math>~\tilde{p}</math>, as [[User:Tohline/SSC/Structure/PolytropesEmbedded#Horedt.27s_Presentation|summarized above]].  (For purposes of simplification, we will for the most part drop the tilde notation.)
<div align="center">
<table border="0" align="center" cellpadding="5">
<tr>
  <td align="right">
<math>~ \frac{1}{R_\mathrm{Horedt}} \cdot \frac{d\tilde{r}}{d \tilde\xi}</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
\frac{d}{d\xi}\biggl[ \tilde\xi ( -\tilde\xi^2 \tilde\theta' )^{(1-n)/(n-3)}\biggr]
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
(-\xi^2\theta^')^{(1-n)/(n-3)}\biggl[ 1 +\frac{(1-n)}{(n-3)} \cdot \xi (-\xi^2\theta^')^{-1} (-\xi^2\theta^')^' \biggr]
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\frac{1}{(n-3)(n+1)}\cdot (-\xi^2\theta^')^{(1-n)/(n-3)} \biggl[ (n-3)(n+1)
+(n-1)\cdot (\theta^')^{-2} \xi f^'
\biggr]
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\frac{(-\xi^2\theta^')^{(1-n)/(n-3)} }{(3-n)(n+1)(\theta^')^{2}}
\biggl[ (3-n)(n+1)(\theta^')^{2} +(1-n) \xi f^' \biggr] \, ;
</math>
  </td>
</tr>
<tr>
  <td align="right">
<math>~ \frac{1}{P_\mathrm{Horedt}} \cdot \frac{d\tilde{p}}{d \tilde\xi}</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
\frac{d}{d\xi}\biggl[ \tilde\theta^{n+1}( -\tilde\xi^2 \tilde\theta' )^{2(n+1)/(n-3)} \biggr]
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
( -\tilde\xi^2 \tilde\theta' )^{2(n+1)/(n-3)}
\biggl[ f^' + f \cdot \frac{2(n+1)}{(n-3)}( -\tilde\xi^2 \tilde\theta' )^{-1} ( -\tilde\xi^2 \tilde\theta' )^' \biggr]
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
( -\tilde\xi^2 \tilde\theta' )^{2(n+1)/(n-3)}
\biggl[ f^' - \frac{2(n+1)}{(n-3)(n+1)} \cdot \frac{f\cdot f^'}{(\theta^')^2} \biggr]
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
\frac{f^' ( -\tilde\xi^2 \tilde\theta' )^{(3n+1)/(n-3)} ( -\tilde\xi^2 \tilde\theta' )^{(1-n)/(n-3)} }{(3-n)(n+1)(\theta^')^2}
\biggl[ (3-n)(n+1)(\theta^')^2 + 2(n+1) \theta^{n+1} \biggr] \, .
</math>
  </td>
</tr>
</table>
</div>
The ratio of these two expressions gives,
<div align="center">
<table border="0" cellpadding="5" align="center">
<tr>
  <td align="right">
<math>~\frac{R_\mathrm{Horedt}}{P_\mathrm{Horedt}} \cdot \frac{dP_e}{dR_\mathrm{eq}}\biggr|_M </math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~f^' ( -\tilde\xi^2 \tilde\theta' )^{(3n+1)/(n-3)} 
\biggl\{ \frac{(3-n)(n+1)(\theta^')^2 + 2(n+1) \theta^{n+1}}{(3-n)(n+1)(\theta^')^{2} +(1-n) \xi f^' } \biggr\} \, ,
</math>
  </td>
</tr>
</table>
</div>
completing our task, as the term inside the curly braces exactly matches the equation excerpt from Horedt's work, as displayed above.
====Kimura's Derivation====
Appreciating that Kimura uses the subscript "1," rather than a tilde, to identify equilibrium parameter values, the requisite expression is equation (22) from [http://adsabs.harvard.edu/abs/1981PASJ...33..299K Kimura's "Paper II,"] as displayed in the following boxed image: 
<div align="center">
<table border="1" align="center" cellpadding="4">
<tr>
  <td align="center">
Excerpts (edited) from [http://adsabs.harvard.edu/abs/1981PASJ...33..299K Kimura (1981b)]
  </td>
</tr>
<tr><td align="left">
<table border="0" align="center">
<tr><td align="center">
[[File:KimuraEq00.png|500px|center|Kimura (1981b) Expressions]]
<!-- [[Image:AAAwaiting01.png|500px|center|Kimura (1981b) Expressions]] -->
</td></tr>
<tr><td align="left">
where,
</td></tr>
<tr><td align="center">
[[File:KimuraEq01.png|500px|center|Kimura (1981b) Expressions]]
<!-- [[Image:AAAwaiting01.png|500px|center|Kimura (1981b) Expressions]] -->
</td></tr>
</table>
</td></tr>
</table>
</div>
Drawing on the additional parameter and variable definitions provided in our [[User:Tohline/SSC/Structure/PolytropesEmbedded#Kimura.27s_Presentation|discussion of Kimura's presentation, above]], we can rewrite this key expression as,
<div align="center">
<table border="0" align="center" cellpadding="5">
<tr>
  <td align="right">
<math>~\frac{R_\mathrm{eq}}{P_e} \cdot \frac{dP_e}{dR_\mathrm{eq}}\biggr|_M ~~\rightarrow ~~ \frac{d\ln{p_1}}{d \ln{r_1}}</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
\frac{v_G \cdot h_G}{k_G} \, ,</math>
  </td>
</tr>
</table>
</div>
where,
<div align="center">
<table border="0" cellpadding="5" align="center">
<tr>
  <td align="right">
<math>~v_G</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\frac{2}{[1-2(n+1)^{-1}]} =\frac{ 2(n+1)}{n-1} \, ,
</math>
  </td>
</tr>
<tr>
  <td align="right">
<math>~u_G</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~(3-1)-\biggl[\frac{1}{1-2(n+1)^{-1}} \biggr] = 2-\frac{(n+1)}{(n-1)} = \frac{(n-3)}{(n-1)} \, ,
</math>
  </td>
</tr>
<tr>
  <td align="right">
<math>~h_G</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
\frac{1}{u_G} \biggl[ \frac{\zeta \theta^n}{\phi^'} \biggr]_1 - \frac{1}{v_G} \biggl[ \frac{\zeta \phi^'}{\theta} \biggr]_1
=
\frac{(n-1)}{(n-3)} \biggl[ \frac{\tilde\xi \tilde\theta^n}{-\tilde\theta^'} \biggr] -
\frac{(n-1)}{2(n+1)} \biggl[ \frac{(n+1)\tilde\xi (-\tilde\theta^')}{\tilde\theta} \biggr]
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~ \frac{(n-1)\tilde\xi}{2(n+1)(n-3)\tilde\theta (-\tilde\theta^')}
\biggl\{ 2(n+1) \tilde\theta^{n+1} + (3-n) (n+1) (-\tilde\theta^')^2 \biggr\} \, ,
</math>
  </td>
</tr>
<tr>
  <td align="right">
<math>~k_G</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~1-
\frac{1}{u_G} \biggl[ \frac{\zeta \theta^n}{\phi^'} \biggr]_1
=1-
\frac{(n-1)}{(n-3)} \biggl[ \frac{\tilde\xi \tilde\theta^n}{-\tilde\theta^'} \biggr]
=
\frac{1}{ (n-3) (-\tilde\theta^') } \biggl\{ (n-3)(- \tilde\theta^') - (n-1) \tilde\xi \tilde\theta^n \biggr\}
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
\frac{1}{ (n+1)(n-3) (-\tilde\theta^')^2 } \biggl\{ (n-3)(n+1) (-\tilde\theta^')^2
- (n-1)\tilde\xi [(n+1) \tilde\theta^n (-\tilde\theta^')] \biggr\} \, .
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
\frac{-1}{ (n+1)(n-3) (-\tilde\theta^')^2 } \biggl\{ (3-n)(n+1) (-\tilde\theta^')^2
+ (1-n)\tilde\xi [(n+1) \tilde\theta^n (\tilde\theta^')] \biggr\} \, .
</math>
  </td>
</tr>
</table>
</div>
Hence, from Kimura's work we find,
<div align="center">
<table border="0" align="center" cellpadding="5">
<tr>
  <td align="right">
<math>~\frac{R_\mathrm{eq}}{P_e} \cdot \frac{dP_e}{dR_\mathrm{eq}}\biggr|_M </math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
\frac{(n+1)\tilde\xi \tilde\theta^'}{\tilde\theta}
\biggl\{ \frac{2(n+1) \tilde\theta^{n+1} + (3-n) (n+1) (\tilde\theta^')^2}{(3-n)(n+1) (\tilde\theta^')^2
+ (1-n)\tilde\xi [(n+1) \tilde\theta^n \tilde\theta^'] } \biggr\} \, .
</math>
  </td>
</tr>
</table>
</div>
Appreciating that <math>~f^' = [(n+1)\tilde\theta^n \tilde\theta^']</math>, we see that the expression inside the curly braces here matches exactly the expression inside the curly braces that was obtained through Horedt's derivation, as it should!  The prefactor is different in the two expressions only because Kimura's result is for a logarithmic derivative whereas Horedt's derivation is not; the ratio of the two prefactors is, simply, the ratio,
<div align="center">
<table border="0" align="center" cellpadding="5">
<tr>
  <td align="right">
<math>~\frac{R_\mathrm{eq}/R_\mathrm{Horedt}}{P_e/P_\mathrm{Horedt}} </math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\frac{\tilde\xi}{\tilde\theta_n^{n+1}}\cdot ( -\tilde\xi^2 \tilde\theta' )^{[(1-n)-2(n+1)]/(n-3)}
</math>
  </td>
</tr>
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\frac{\tilde\xi}{\tilde\theta_n^{n+1}}\cdot ( -\tilde\xi^2 \tilde\theta' )^{-(3n+1)/(n-3)} \, .
</math>
  </td>
</tr>
</table>
</div>
In a [[User:Tohline/SSC/Virial/PolytropesEmbedded/SecondEffortAgain#KimuraApplication|separate discussion]], specifically focused on the <math>~n=5</math> mass-radius relationship, we show how Kimura's analysis of turning points can be usefully applied.
====Location of Pressure Limit====
Now we can identify the location along the M<sub>1</sub> sequence where the turning point set by <math>~P_\mathrm{max}</math> occurs by setting the numerator of this expression equal to zero, specifically,
<div align="center">
<table border="0" align="center" cellpadding="5">
<tr>
  <td align="right">
<math>~2(n+1) \tilde\theta^{n+1} + (3-n) (n+1) (\tilde\theta^')^2 </math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
0 \, .
</math>
  </td>
</tr>
</table>
</div>
This means that the equilibrium model that sits at the <math>~P_\mathrm{max}</math> turning point will have,
<div align="center">
<table border="0" align="center" cellpadding="5">
<tr>
  <td align="right">
<math>~\frac{\tilde\theta^{n+1}}{(\tilde\theta^')^2} </math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~
\frac{(n-3)}{2} \, .
</math>
  </td>
</tr>
</table>
</div>
===Other Limits===
In a similar fashion, [http://adsabs.harvard.edu/abs/1981PASJ...33..299K Kimura (1981b)] derived mathematical expressions that identify the location of other turning points along equilibrium sequences of bounded polytropic configurations.  An M<sub>1</sub> sequence &#8212; as displayed, for example, in the set of P-R diagrams shown in [[User:Tohline/SSC/Structure/PolytropesEmbedded#WhitworthFig1b|Figure 1, above]] &#8212; exhibits not only an "extremal of p<sub>1</sub>" but also an "extremal of r<sub>1</sub>."  As we have [[User:Tohline/SSC/Structure/PolytropesEmbedded#Location_of_Pressure_Limit|just reviewed]], the first of these is identified by setting <math>~(d\ln p_1/d\ln r_1)_{M} = 0</math> or, using Kimura's more compact terminology, the first occurs at a location that satisfies the condition,
<div align="center">
<math>h_G = 0 \, ,</math> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
that is, where &hellip; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<math>~\tilde\theta^{n+1} (\tilde\theta^')^{-2} = (n-3)/2 \, .</math>
</div>
Similarly, Kimura points out that an "extremal in r<sub>1</sub>" along an M<sub>1</sub> sequence occurs at a location that satisfies the condition,
<div align="center">
<math>k_G = 0 \, ,</math> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
that is, where &hellip; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
<math>~\tilde\xi \tilde\theta^{n} (-\tilde\theta^')^{-1} = (n-3)/(n-1) \, .</math>
</div>
As is illustrated by the plots presented in [[User:Tohline/SSC/Structure/PolytropesEmbedded#Stahler1983Fig17|Figure 2, above]], turning points also arise in the mass-radius relationship of bounded polytropic configurations having <math>~n > 3</math>.  These are identified by Kimura as "p<sub>1</sub> sequences" because the external pressure is held fixed while the system's mass and corresponding equilibrium radius is varied.  In &sect;5 of his [http://adsabs.harvard.edu/abs/1981PASJ...33..299K "Paper II,"] Kimura points out that the same two conditions &#8212; namely, <math>~h_G = 0</math> and <math>~k_G = 0</math> &#8212; also identify the location of extrema in M<sub>1</sub> along, respectively, p<sub>1</sub> sequences and r<sub>1</sub> sequences.


====Overlap with Stahler's Presentation====
We can also identify extrema in r<sub>1</sub> along p<sub>1</sub> sequences by setting <math>~(\dot{p}_1/p_1) = 0</math> in [http://adsabs.harvard.edu/abs/1981PASJ...33..299K Kimura's] equation (17), then substituting the resulting expression for the function <math>~Z</math>, namely,
We can invert the above expression for <math>~P_e(K,M)</math> to obtain the following expression for <math>~M(K,P_e)</math>:
<div align="center">
<div align="center">
<math>M^{6}= \biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 P_e G^9} \biggr) \frac{\xi_e^{18}}{(3 + \xi_e^2)^{12}} </math> .
<math>~Z = v_1 \, ,</math>
</div>
</div>
 
into his equations (15) and (16).  The ratio of these two resulting expressions gives,
If, following Stahler's lead, we normalize this expression by <math>~M_\mathrm{SWS}</math> (evaluated for <math>~n=5</math>) and we normalize the above expression for <math>~R_\mathrm{eq}</math> by <math>~R_\mathrm{SWS}</math> (evaluated for <math>~n=5</math>), we obtain,
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<table border="0" align="center" cellpadding="5">
 
<tr>
  <td align="right">
<math>
\frac{M}{M_\mathrm{SWS}}
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 P_e G^9} \biggr)^{1/6} \frac{\xi_e^{3}}{(3 + \xi_e^2)^{2}}
\biggl[ \biggl( \frac{2\cdot 3}{5G} \biggr)^{3/2} K^{5/3} P_\mathrm{ex}^{-1/6}  \biggr]^{-1}
</math>
  </td>
</tr>
 
<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\biggr( \frac{3^{2} \cdot 5^3 }{4\pi } \biggr)^{1/2} \frac{\xi_e^{3}}{(3 + \xi_e^2)^{2}}  \, ,
</math>
  </td>
</tr>


<tr>
<tr>
   <td align="right">
   <td align="right">
<math>
<math>~\frac{d\ln M_1}{d \ln r_1}\biggr|_{p_1}</math>
\frac{R_\mathrm{eq}}{R_\mathrm{SWS}}
</math>
   </td>
   </td>
   <td align="center">
   <td align="center">
<math>~=~</math>
<math>~=</math>
   </td>
   </td>
   <td align="left">
   <td align="left">
<math>
<math>~
\biggl[ \frac{\pi M^4 G^5}{2^3 \cdot 3^7 K^5} \biggr]^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5}
\frac{u_1 -(u_G/v_G)v_1}{1 - v_1/v_G}
\biggl[  \biggl( \frac{2\cdot 3}{5G} \biggr)^{1/2} K^{5/6} P_\mathrm{ex}^{-1/3}
=
\biggr]^{-1}  
[u_1 v_G - u_G v_1][v_G - v_1]^{-1}
</math>
</math>
   </td>
   </td>
Line 1,402: Line 2,512:
   </td>
   </td>
   <td align="center">
   <td align="center">
<math>~=~</math>
<math>~=</math>
   </td>
   </td>
   <td align="left">
   <td align="left">
<math>
<math>~
\biggl[ \biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 P_e G^9} \biggr)^{1/3} \frac{\xi_e^{6}}{(3 + \xi_e^2)^{4}} \biggr]
\biggl[\frac{2(n+1)}{(n-1)} \cdot \frac{\xi \theta^n}{(-\theta^')} - \frac{(n-3)}{(n-1)} \cdot \frac{(n+1)\xi (-\theta^')}{\theta} \biggr]  
\biggl[ \frac{\pi G^5}{2^3 \cdot 3^7 K^5} \biggr]^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5}
\biggl[\frac{2(n+1)}{(n-1)} - \frac{(n+1)\xi (-\theta^')}{\theta} \biggr]^{-1}
\biggl( \frac{5G}{2\cdot 3} \biggr)^{1/2} \biggl[  K^{-5/6} P_\mathrm{ex}^{1/3} \biggr]
</math>
</math>
   </td>
   </td>
Line 1,418: Line 2,527:
   </td>
   </td>
   <td align="center">
   <td align="center">
<math>~=~</math>
<math>~=</math>
   </td>
   </td>
   <td align="left">
   <td align="left">
<math>
<math>~
\biggr( \frac{3^{2} \cdot 5}{2^2 \pi } \biggr)^{1/2\frac{\xi_e}{(3 + \xi_e^2)} \, .
\frac{\xi }{(-\theta^')} \biggl[ \frac{2 \theta^{n+1} - (n-3) (-\theta^')^2 }{2\theta - (n-1)\xi (-\theta^') } \biggr]
</math>
</math>
   </td>
   </td>
Line 1,428: Line 2,537:
</table>
</table>
</div>
</div>
This set of parametric relations that relate the mass of the truncated configuration to its radius via the parameter, <math>~\xi_e</math>, has been recorded to the immediate right of Stahler's name in our [[User:Tohline/SSC/Structure/PolytropesEmbedded#n5Summary|<math>~n=5</math> summary table]], below. 


Stahler points out (see his equation B13) that, for this particular pressure-bounded polytropic sequence, <math>~\xi_e</math> can be eliminated between the expressions to obtain the following direct algebraic relationship between <math>~M</math> and <math>~R_\mathrm{eq}</math>:
<span id="TurningPointXmax">As has just been reviewed, the condition <math>~h_G=0</math> results from setting the numerator of this expression equal to zero and identifies extrema in M<sub>1</sub> along p<sub>1</sub> sequences.  In addition, now, we can identify the condition for extrema in r<sub>1</sub> along p<sub>1</sub> sequences by setting the denominator to zero.</span> The condition is,
<div align="center">
<table border="0" cellpadding="3">
<tr>
  <td align="right">
<math>
\biggl( \frac{M}{M_\mathrm{SWS}} \biggr)^2 - 5 \biggl( \frac{M}{M_\mathrm{SWS}} \biggr)\biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)
+ \frac{20\pi}{3} \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)^4
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
~0 \, .
</math>
  </td>
</tr>
</table>
</div>
Viewed as a quadratic equation in the mass, the roots of this expression give,
<div align="center">
<div align="center">
<table border="0" cellpadding="3">
<math>~\frac{\xi (-\theta^')}{\theta} = \frac{2}{(n-1)} \, .</math>
<tr>
  <td align="right">
<math>
\frac{M}{M_\mathrm{SWS}}
</math>
  </td>
  <td align="center">
<math>~=~</math>
  </td>
  <td align="left">
<math>
\frac{5}{2} \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr) \biggl\{ 1 \pm \biggl[ 1 - \frac{16\pi}{15}
\biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)^2 \biggr]^{1/2}  \biggr\} \, .
</math>
  </td>
</tr>
</table>
</div>
</div>
[<font color="red">CORRECTION:  Changed factor inside square root from <math>~16\pi/3</math> to <math>~16\pi/15</math> on 24 December 2014.</font>]  We have used this expression to generate the complete <math>~n=5</math> sequence shown here in the top panel of Figure 2 &#8212; the solid green segment of the curve shows the negative root and the solid red segment of the curve was generated using the positive root.


ASIDE:  In his Appendix B, [http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler (1983)] claims that the quadratic equation relating <math>~M</math> directly to <math>~R_\mathrm{eq}</math> (his equation B13) can be obtained by analytically integrating the first-order ordinary differential equation presented as his equation B10.  I don't think that this is possible without knowing ahead of time how <math>~M</math> relates to <math>~R_\mathrm{eq}</math> through the above-derived parametric relations in <math>~\xi_e</math>.
===Some Tabulated Values===


[<font color="red">29 September 2014</font> by J. E. Tohline] Now that (I think) I've finished deriving the properly defined [[User:Tohline/SSC/Virial/Polytropes#Nonrotating_Adiabatic_Configuration_Embedded_in_an_External_Medium|virial equilibrium condition for embedded polytropes]] and have reconciled that equilibrium expression with Horedt's corresponding specification of the equilibrium radius and surface-pressure, it's time to [[User:Tohline/SSC/Structure/StahlerMassRadius|revisit the concern]] that was expressed in this "ASIDE" regarding the mass-radius relationship for embedded, <math>~n=5</math> polytropes presented by Stahler.
<div align="center" id="Table3">
 
<table border="1" cellpadding="8" align="center">
===Tabular Summary (n=5) ===
<tr><th align="center" colspan="14">Table 3: &nbsp; Turning-Point Locations along M-R Sequences of Pressure-Truncated Polytropes</th></tr>
<span id="n5Summary">
<div align="center">
<table border="1" cellpadding="8" width="95%">
<tr>
<tr>
   <th align="center" colspan="3">
   <td align="center" rowspan="2">n</td>
Properties of <math>~n=5</math> Polytropes Embedded in an External Medium of Pressure <math>~P_e</math>  
  <td align="center" colspan="6"><font color="yellow" size="+2">&#x25CF;</font> Maximum Radius <font color="yellow" size="+2">&#x25CF;</font></td>
<br>
  <td align="center" colspan="6"><font color="darkgreen" size="+2">&#x25CF;</font> Maximum Mass <font color="darkgreen" size="+2">&#x25CF;</font></td>
(and, accordingly, truncated at radius <math>~\xi_e</math>)
  </th>
</tr>
</tr>
<tr>
<tr>
   <td align="center" colspan="3">
   <td align="center"><math>~\tilde\xi</math></td>
<table border="0" cellpadding="3">
  <td align="center"><math>~\tilde\theta</math></td>
<tr>
  <td align="center"><math>~\biggl|\frac{d\theta}{d\xi}\biggr|_\tilde\xi</math></td>
   <td align="right">
   <td align="center"><math>~\frac{(n-1)}{2}\biggl[ \frac{\xi}{\theta} \biggl|\frac{d\theta}{d\xi}\biggr|~\biggr]_\tilde\xi</math></td>
<math>
  <td align="center"><math>~\frac{R}{R_\mathrm{SWS}}</math>
~\theta_5 = \biggl( 1 + \frac{\xi_e^2}{3} \biggr)^{-1/2}
  <td align="center"><math>~\frac{M}{M_\mathrm{SWS}}</math>
</math>
   <td align="center"><math>~\tilde\xi</math></td>
   </td>
   <td align="center"><math>~\tilde\theta</math></td>
 
   <td align="center"><math>~\biggl|\frac{d\theta}{d\xi}\biggr|_\tilde\xi</math></td>
   <td align="center">
  <td align="center"><math>~\frac{(n-3)}{2}\biggl[ \frac{1}{\theta^{n+1}} \biggl(\frac{d\theta}{d\xi}\biggr)^2 \biggr]_\tilde\xi</math></td>
&nbsp; &nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; &nbsp;
  <td align="center"><math>~\frac{R}{R_\mathrm{SWS}}</math>
  </td>
   <td align="center"><math>~\frac{M}{M_\mathrm{SWS}}</math>
 
   <td align="right">
<math>
~\frac{d\theta_5}{d\xi} \biggr|_{\xi_e} = - \frac{\xi_e}{3} \biggl( 1 + \frac{\xi_e^2}{3} \biggr)^{-3/2}
</math>
   </td>
</tr>
</tr>
</table>
  </td>
</tr>
<tr>
<tr>
   <td align="center" rowspan="1">
   <td align="center">3</td>
[http://adsabs.harvard.edu/abs/1970MNRAS.151...81H Horedt (1970)]
  <td align="center">2.172</td>
<br>for<br>
  <td align="center">0.5387</td>
fixed <math>~(M,K_n)</math>
  <td align="center">0.2496</td>
   </td>
   <td align="center">1.006</td>
   <td align="center">
   <td align="center">0.5717</td>
<math>
  <td align="center">1.726</td>
~r_a = \frac{R_\mathrm{eq}}{R_\mathrm{Horedt}} = \biggl\{ 3 \biggl[ \frac{(\xi_e^2/3)^5}{(1+\xi_e^2/3)^{6}} \biggr] \biggr\}^{-1/2}
  <td align="center">6.89684862</td>
</math>
  <td align="center">0.0</td>
   </td>
   <td align="center">-0.04242976</td>
   <td align="center">
   <td align="center">--</td>
<math>
  <td align="center">0.0</td>
~p_a = \frac{P_e}{P_\mathrm{Horedt}} = 3^3 \biggl[ \frac{(\xi_e^2/3)^3}{(1+\xi_e^2/3)^{4}} \biggr]^3
   <td align="center">2.9583456</td>
</math>
   </td>
</tr>
</tr>
<tr>
<tr>
   <td align="center" rowspan="1">
   <td align="center">3.05</td>
[http://adsabs.harvard.edu/abs/1981MNRAS.195..967W Whitworth (1981)]
  <td align="center">2.162</td>
<br>for<br>
  <td align="center">0.5437</td>
fixed <math>~(M,K_n)</math>
  <td align="center">0.2479</td>
   </td>
   <td align="center">1.010</td>
   <td align="center">
   <td align="center">0.5704</td>
<math>
  <td align="center">1.715</td>
\frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl\{ \frac{2^{15}}{5^5} \biggl[ \frac{(\xi_e^2/3)^5}{(1+\xi_e^2/3)^{6}} \biggr] \biggr\}^{-1/2}
  <td align="center">5.034</td>
</math>
   <td align="center">0.1152</td>
   </td>
   <td align="center">0.07842</td>
   <td align="center">
  <td align="center">0.973</td>
<math>
  <td align="center">0.2707</td>
\frac{P_e}{P_\mathrm{rf}} = \frac{2^{29}}{5^9} \biggl[ \frac{(\xi_e^2/3)^3}{(1+\xi_e^2/3)^{4}} \biggr]^3
   <td align="center">2.829</td>
</math>
   </td>
</tr>
</tr>
<tr>
<tr>
   <td align="center" rowspan="2">
   <td align="center">3.5</td>
[http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler (1983)]
  <td align="center">2.050</td>
<br>for<br>
  <td align="center">0.5930</td>
fixed <math>~(P_e,K_n)</math>
  <td align="center">0.2340</td>
   </td>
   <td align="center">1.011</td>
   <td align="center">
   <td align="center">0.5630</td>
<math>
  <td align="center">1.594</td>
\frac{R_\mathrm{eq}}{R_\mathrm{SWS}} = \biggl\{ \frac{3\cdot 5}{2^2 \pi} \biggl[ \frac{\xi_e^2/3}{(1+\xi_e^2/3)^{2}} \biggr] \biggr\}^{1/2}
  <td align="center">3.910</td>
</math>
  <td align="center">0.2788</td>
   </td>
   <td align="center">0.1126</td>
   <td align="center">
   <td align="center">0.994</td>
<math>
  <td align="center">0.4180</td>
\frac{M}{M_\mathrm{SWS}} = \biggl[  \biggl( \frac{3 \cdot 5^3}{2^2\pi} \biggr) \frac{(\xi_e^2/3)^3}{(1+\xi_e^2/3)^{4}} \biggr]^{1/2}
   <td align="center">2.311</td>
</math>
   </td>
</tr>
</tr>
<tr>
<tr>
   <td align="center" colspan="2">
   <td align="center">5</td>
<math>
  <td align="center"><math>~\sqrt{3}</math></td>
\biggl( \frac{M}{M_\mathrm{SWS}} \biggr)^2 - 5 \biggl( \frac{M}{M_\mathrm{SWS}} \biggr)\biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)
  <td align="center"><math>~\frac{1}{\sqrt{2}}</math></td>
+ \frac{2^2 \cdot 5 \pi}{3} \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)^4 = 0
  <td align="center"><math>~\frac{1}{\sqrt{24}}</math></td>
</math>
  <td align="center"><math>~1</math></td>
  </td>
  <td align="center"><math>~\biggl( \frac{3\cdot 5}{2^4\pi}\biggr)^{1 / 2}</math></td>
  <td align="center"><math>~\biggl( \frac{3\cdot 5^3}{2^6\pi}\biggr)^{1 / 2}</math></td>
  <td align="center"><math>~3</math></td>
  <td align="center"><math>~\frac{1}{2}</math></td>
  <td align="center"><math>~\frac{1}{8}</math></td>
  <td align="center"><math>~1</math></td>
  <td align="center"><math>~\biggl( \frac{3^2\cdot 5}{2^6\pi}\biggr)^{1 / 2}</math></td>
  <td align="center"><math>~\biggl( \frac{3^4\cdot 5^3}{2^{10}\pi}\biggr)^{1 / 2}</math></td>
</tr>
</tr>
<tr>
<tr>
   <td align="left" colspan="3">
   <td align="center">6</td>
NOTE:  None of the analytic expressions for the dimensionless radius, pressure, or mass presented in this table explicitly appear in the referenced articles by Horedt, by Whitworth, or by Stahler but, as is discussed fully above, they are straightforwardly derivable from the more general relations that appear in these papers. The final polynomial relating the dimensionless mass to the dimensionless radius ''does'' explicitly appear as equation (B13) in [http://adsabs.harvard.edu/abs/1983ApJ...268..165S Stahler (1983)].
  <td align="center">1.6</td>
 
  <td align="center">0.7510</td>
Additional discussion of Stahler's analytic mass-radius relation is presented in an  [[User:Tohline/SSC/Virial/PolytropesEmbedded/SecondEffortAgain#Plotting_Stahler.27s_Relation|accompanying chapter]].
  <td align="center">0.1884</td>
   </td>
  <td align="center">1.003</td>
  <td align="center">0.5404</td>
  <td align="center">1.301</td>
  <td align="center">2.7</td>
  <td align="center">0.5811</td>
  <td align="center">0.1221</td>
  <td align="center">0.999</td>
  <td align="center">0.4802</td>
   <td align="center">1.635</td>
</tr>
</tr>
</table>
</table>
</div>
</div>
</span>


=Related Discussions=
=Related Discussions=

Latest revision as of 16:40, 16 April 2017

Embedded Polytropic Spheres

LSU Structure still.gif

In a separate discussion we have shown how to determine the structure of isolated polytropic spheres. These are rather idealized stellar structures in which the pressure and density both drop to zero at the surface of the configuration. Here we consider how the equilibrium radius of a polytropic configuration of a given <math>~M</math> and <math>~K_\mathrm{n}</math> is modified when it is embedded in an external medium of pressure <math>~P_e</math>. We will begin by reviewing the general properties of embedded (and truncated) polytropes for a wide range of polytropic indexes, principally summarizing the published descriptions provided by Horedt (1970), by Whitworth (1981), by Kimura (1981a), and by Stahler (1983). Then we will focus in more detail on polytropes of index <math>~n</math> = 1 and <math>~n</math> = 5 because their structures can be described by closed-form analytic expressions.


Whitworth's (1981) Isothermal Free-Energy Surface
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General Properties

Horedt's Presentation

It appears as though Horedt (1970) was the first to draw an analogy between the mass limit that is associated with bounded isothermal spheres — the so-called Bonnor-Ebert spheres — and the limiting mass that can be found in association with equilibrium sequences of embedded polytropes that have polytropic indexes <math>~n > 3</math>. Using a tilde to denote values of parameters at the (truncated) edge of a pressure-bounded polytropic sphere, Horedt (see the bottom of his p. 83) derives the following set of parametric equations relating the configuration's dimensionless radius, <math>~r_a</math>, to a specified dimensionless bounding pressure, <math>~p_a</math>:

<math> ~r_a \equiv \frac{R_\mathrm{eq}}{R_\mathrm{Horedt}} </math>

<math>~=~</math>

<math> \tilde\xi ( -\tilde\xi^2 \tilde\theta' )^{(1-n)/(n-3)} \, , </math>

<math> ~p_a \equiv \frac{P_\mathrm{e}}{P_\mathrm{Horedt}} </math>

<math>~=~</math>

<math> \tilde\theta_n^{n+1}( -\tilde\xi^2 \tilde\theta' )^{2(n+1)/(n-3)} \, , </math>

where it is understood that, as discussed elsewhere, <math>~\theta_n(\xi)</math> is the solution to the Lane-Emden equation for a polytrope of index <math>~n</math>,

<math> \tilde\theta' </math>

<math>~\equiv~</math>

<math> \frac{d\theta_n}{d\xi} ~~~\mathrm{evaluated}~\mathrm{at}~\tilde\xi \, , </math>

<math> ~R_\mathrm{Horedt} </math>

<math>~\equiv~</math>

<math> \alpha_r \biggl( \frac{\alpha_M}{M} \biggr)^{(1-n)/(n-3)} = \biggl[ \frac{4\pi}{(n+1)^n}\biggl( \frac{G}{K_n} \biggr)^n M^{n-1} \biggr]^{1/(n-3)} \, , </math>

<math> ~P_\mathrm{Horedt} </math>

<math>~\equiv~</math>

<math> K_n \biggl( \frac{\alpha_M}{M} \biggr)^{2(n+1)/(n-3)} = K_n^{4n/(n-3)}\biggl[ \frac{(n+1)^3}{4\pi G^3 M^2} \biggr]^{(n+1)/(n-3)} \, . </math>

Notice that, via these normalizations, Horedt chose to express <math>~R_\mathrm{eq}</math> and <math>~P_\mathrm{e}</math> in terms of <math>~K_\mathrm{n}</math> and the system's total mass, <math>~M</math>.

Whitworth's Presentation

In §5 of his paper, Whitworth (1981) also presents the set of parametric equations that define what the equilibrium radius, <math>~R_\mathrm{eq}</math>, is of an embedded polytrope for a certain imposed external pressure, <math>~P_\mathrm{e}</math>, namely,

<math> ~R_\mathrm{eq} </math>

<math>~=~</math>

<math> ~R_\mathrm{rf} \biggl\{ \frac{4\eta}{5|\eta-1|} \biggl(\frac{\xi}{3} \biggr)^\eta \biggl|\frac{d\theta_n}{d\xi} \biggr|^{(2-\eta)} \biggr\}_{\xi_e}^{1/(3\eta - 4)} </math>

<math> \Rightarrow ~~~~~ \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{rf}} \biggr)^{(3-n)} </math>

<math>~=~</math>

<math> \biggl[ \frac{4(n+1)}{5} \biggr]^{n} \biggl(\frac{\xi_e}{3} \biggr)^{(n+1)} \biggl|\frac{d\theta_n}{d\xi} \biggr|^{(n-1)}_{\xi_e} \, , </math>

<math> ~P_\mathrm{e} </math>

<math>~=~</math>

<math> ~P_\mathrm{rf} \biggl\{ 2^{-8/\eta} \biggl(\frac{5|\eta-1|}{\eta} \biggr)^3 \biggl(\frac{3}{\xi} \biggr)^4 \biggl|\frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}_{\xi_e}^{\eta/(3\eta - 4)} \theta_n^{\eta/(\eta-1)} </math>

<math> \Rightarrow ~~~~~ \biggl( \frac{P_\mathrm{e}}{P_\mathrm{rf}} \biggr)^{(3-n)} </math>

<math>~=~</math>

<math> 2^{-8n}\biggl\{ \biggl(\frac{5}{n+1} \biggr)^3 \biggl(\frac{3}{\xi} \biggr)^4 \theta_n^{(3-n)} \biggl|\frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}_{\xi_e}^{(n+1)} \, , </math>

where, in order to obtain the second line of the two relations we have used the substitution, <math>~\eta \rightarrow (1+1/n)</math>, and, as is detailed in an accompanying ASIDE, Whitworth "referenced" <math>~P_\mathrm{e}</math> and <math>~R_\mathrm{eq}</math> to, respectively,

<math> P_\mathrm{rf}^{(4-3\eta)} </math>

<math>~=~</math>

<math> 2^{-2(4+\eta)} \biggl( \frac{3^4 \cdot 5^3}{\pi} \biggr)^\eta \biggl[ \frac{K_n^4}{G^{3\eta} M^{2\eta} } \biggr] </math>

<math> \Rightarrow ~~~~~ P_\mathrm{rf}^{(n-3)} </math>

<math>~=~</math>

<math> 2^{-2(5n+1)} \biggl( \frac{3^4 \cdot 5^3}{\pi} \biggr)^{(n+1)} \biggl[ \frac{K_n^{4n}}{G^{3(n+1)} M^{2(n+1)} } \biggr] \, , </math>

<math> ~R_\mathrm{rf}^\eta </math>

<math>~=~</math>

<math> \frac{2^2}{K_n} \biggl(\frac{GM}{3\cdot 5}\biggr)^\eta P_\mathrm{rf}^{(1-\eta)} </math>

<math> \Rightarrow ~~~~~ R_\mathrm{rf}^{(n+1)} </math>

<math>~=~</math>

<math> \biggl( \frac{2^2}{K_n} \biggr)^{n} \biggl(\frac{GM}{3\cdot 5}\biggr)^{(n+1)} P_\mathrm{rf}^{-1} </math>

<math> \Rightarrow ~~~~~ R_\mathrm{rf}^{(3-n)} </math>

<math>~=~</math>

<math> \biggl( \frac{2^2}{K_n} \biggr)^{n(3-n)/(n+1)} \biggl(\frac{GM}{3\cdot 5}\biggr)^{(3-n)} P_\mathrm{rf}^{(n-3)/(n+1)} </math>

 

<math>~=~</math>

<math> \biggl( \frac{2^2}{K_n} \biggr)^{n(3-n)/(n+1)} \biggl(\frac{GM}{3\cdot 5}\biggr)^{(3-n)} \biggl\{2^{-2(5n+1)} \biggl( \frac{3^4 \cdot 5^3}{\pi} \biggr)^{(n+1)} \biggl[ \frac{K_n^{4n}}{G^{3(n+1)} M^{2(n+1)} } \biggr] \biggr\}^{1/(n+1)} </math>

 

<math>~=~</math>

<math>K_n^{n} ( 2^2 )^{-(n+1)} \biggl(\frac{GM}{3\cdot 5}\biggr)^{(3-n)} \biggl( \frac{3^4 \cdot 5^3}{\pi} \biggr) \biggl[ \frac{1}{G^{3} M^{2} } \biggr] </math>

 

<math>~=~</math>

<math> 2^{-2(n+1)} \pi^{-1} 3^{n+1} \cdot 5^{n} K_n^n G^{-n} M^{1-n} </math>

Via these normalizations, Whitworth — as did Horedt (1970) — chose to express <math>~R_\mathrm{eq}</math> and <math>~P_\mathrm{e}</math> in terms of <math>~K_\mathrm{n}</math> and the system's total mass, <math>~M</math>.

To convert from Whitworth's expressions, which use one set of normalization parameters <math>~(R_\mathrm{rf},P_\mathrm{rf})</math>, to Horedt's expressions, which use a somewhat different set of normalization parameters — identified here as <math>~(R_\mathrm{Horedt},P_\mathrm{Horedt})</math> — one simply needs to make use of the relations,

<math> \biggl( \frac{R_\mathrm{rf}}{R_\mathrm{Horedt}} \biggr)^{(3-n)} </math>

<math>~=~</math>

<math> 3^{(n+1)} \biggl[ \frac{5}{2^2 (n+1)} \biggr]^{n} \, . </math>

<math> \biggl( \frac{P_\mathrm{rf}}{P_\mathrm{Horedt}} \biggr)^{(3-n)} </math>

<math>~=~</math>

<math> 2^{8n} \biggl[ \frac{(n+1)^3}{3^4 \cdot 5^3} \biggr]^{(n+1)} \, , </math>

Kimura's Presentation

At the same time Whitworth's work was being published, Kimura (1981a) also published a derivation of the equations that define the equilibrium properties of embedded, pressure-truncated polytropic configurations. (Note that an erratum has been published correcting typographical errors that appear in a few equations of the original paper.) When compared with, for example, Horedt's published work — which Kimura references — Kimura's set of structural equations are a bit more difficult to digest because they include (a) an equation-of-state index that is different from the traditional polytropic index — specifically (see his equation 6),

<math>~\sigma \equiv (n+1)^{-1} \, </math>

— which was Kimura's effort to more gracefully accommodate discussions of isothermal <math>~(n=\infty)</math> configurations; and (b) an additional integer index, <math>~m</math>, so that a single set of equations can be used to specify the structure of planar <math>~(m = 1)</math> and cylindrical <math>~(m=2)</math> as well as spherical <math>~(m=3)</math> configurations. In the present context, we will fix the value to <math>~m = 3</math>. Kimura also chose to express his structural solutions in terms of a dimensionless radius, <math>~\zeta</math>, instead of the traditional variable, <math>~\xi</math> — note that the two are related via the expression,

<math>~\zeta = (n+1)^{1/2} \xi \, ;</math>

and in terms of a dimensionless gravitational potential, <math>~\phi</math>, instead of the traditional dimensionless enthalpy variable, <math>~\theta_n</math> — note that the two are related via the expression (see Kimura's equation 12),

<math>~\phi = \sigma^{-1}(1 - \theta_n) \, .</math>

Given this relationship, we note as well that,

<math>~\phi^' \equiv \frac{d\phi}{d\zeta} = -\frac{d\theta_n}{d\xi} \cdot \biggl[ \sigma^{-1} \frac{d\xi}{d\zeta} \biggr] = -\frac{d\theta_n}{d\xi} (n+1)^{1/2} \, .</math>

The set of equilibrium equations derived by Kimura (1981a) in what he identifies as "Paper I" — see especially his equations number (16) and (23) — are summarized most succinctly in Table 1 of his "Paper II" (Kimura 1981b). The equations he presents for "radial distance," "pressure," and "fractional mass within <math>~\tilde{\zeta}</math>" are, respectively,

<math> ~\frac{R_\mathrm{eq}}{R_\mathrm{Kimura}} </math>

<math>~=~</math>

<math> \tilde\zeta = (n+1)^{1/2} \tilde\xi \, , </math>

<math> ~\frac{P_\mathrm{e}}{P_\mathrm{Kimura}} </math>

<math>~=~</math>

<math> \tilde\theta_n^{n+1}\, , </math>

<math> ~\frac{M}{M_\mathrm{Kimura}} </math>

<math>~=~</math>

<math> \tilde\zeta^2 {\tilde\phi}^' = (n+1)^{3/2} \biggl[ - \xi^2 \frac{d\theta_n}{d\xi} \biggr]_{\tilde\xi} \, , </math>

where, expressed in terms of the central pressure, <math>~p_*</math>, and the polytropic constant, <math>~K_n, ~[</math>note that, in Kimura's paper, <math>~H = K_n^{n/(n+1)}]</math>, the relevant normalization parameters are,

<math> ~R_\mathrm{Kimura} </math>

<math>~\equiv~</math>

<math> (4\pi G)^{-1/2} H p_*^{\sigma - 1/2} = (4\pi G)^{-1/2} K_n^{n/(n+1)} p_*^{(1-n)/[2(n+1)]} \, , </math>

<math> ~P_\mathrm{Kimura} </math>

<math>~=~</math>

<math> p_* \, , </math>

<math> ~M_\mathrm{Kimura} </math>

<math>~=~</math>

<math> (4\pi G)^{-3/2} (4\pi) H^2 p_*^{2\sigma - 1/2} = (4\pi G^3)^{-1/2} K_n^{2n/(n+1)} p_*^{(3-n)/[2(n+1)]} \, . </math>

In order to compare Kimura's equilibrium expressions for <math>~R_\mathrm{eq}</math> and <math>~P_e</math> with the corresponding expressions presented by Horedt and by Whitworth, we need to replace <math>~p_*</math> by <math>~M</math> in both expressions. Inverting Kimura's expression for <math>~M</math>, we have,

<math> ~p_*^{(3-n)/[2(n+1)]} </math>

<math>~=~</math>

<math> M (n+1)^{-3/2}( - \tilde\xi^2 \tilde\theta^' )^{-1} (4\pi G^3)^{1/2} K_n^{-2n/(n+1)} \, . </math>

Hence,

<math> ~P_\mathrm{Kimura} </math>

<math>~=~</math>

<math> [ M (n+1)^{-3/2}( - \tilde\xi^2 \tilde\theta^' )^{-1} (4\pi G^3)^{1/2} K_n^{-2n/(n+1)} ]^{2(n+1)/(3-n)} \, , </math>

 

<math>~=~</math>

<math> [ M^{-2} (n+1)^{3}( - \tilde\xi^2 \tilde\theta^' )^{2} (4\pi G^3)^{-1} K_n^{4n/(n+1)} ]^{(n+1)/(n-3)} \, , </math>

 

<math>~=~</math>

<math>~P_\mathrm{Horedt} [ ( - \tilde\xi^2 \tilde\theta^' )^{2} ]^{(n+1)/(n-3)} </math>

<math>\Rightarrow ~~~~~ P_e</math>

<math>~=~</math>

<math>~P_\mathrm{Horedt} ~\tilde\theta^{n+1} ( - \tilde\xi^2 \tilde\theta^' )^{2(n+1)/(n-3)} \, , </math>

which matches Horedt's expression for <math>~P_e</math>. Also after replacement we obtain,

<math> ~R_\mathrm{Kimura} </math>

<math>~=~</math>

<math> (4\pi G)^{-1/2} K^{n/(n+1)} [ M (n+1)^{-3/2}( - \tilde\xi^2 \tilde\theta^' )^{-1} (4\pi G^3)^{1/2} K_n^{-2n/(n+1)} ]^{(1-n)/(3-n)} </math>

 

<math>~=~</math>

<math> M^{(n-1)/(n-3)} ( - \tilde\xi^2 \tilde\theta^' )^{(1-n)/(n-3)} (n+1)^{3(1-n)/2(n-3)} (4\pi)^{[(1-n)-(3-n)]/[2(3-n)]} G^{[3(1-n)- (3-n)]/[2(3-n)]} [ K_n^{n(3-n)-2n(1-n)} ]^{1/[(n+1)(3-n)]} </math>

 

<math>~=~</math>

<math> M^{(n-1)/(n-3)} ( - \tilde\xi^2 \tilde\theta^' )^{(1-n)/(n-3)} (n+1)^{3(1-n)/2(n-3)} (4\pi)^{1/(n-3)} G^{n/(n-3)} K_n^{-n/(n-3)} </math>

<math>~\Rightarrow ~~~~~ R_\mathrm{eq}</math>

<math>~=~</math>

<math>\tilde\xi( - \tilde\xi^2 \tilde\theta^' )^{(1-n)/(n-3)} (n+1)^{[3(1-n)+(n-3)]/2(n-3)} \biggl[ 4\pi \biggl( \frac{G}{K_n} \biggr)^{n} M^{(n-1)} \biggr]^{1/(n-3)} </math>

 

<math>~=~</math>

<math> R_\mathrm{Horedt}~ \tilde\xi( - \tilde\xi^2 \tilde\theta^' )^{(1-n)/(n-3)} \, , </math>

which exactly matches Horedt's expression for <math>~R_\mathrm{eq}</math>.

Stahler's Presentation

Similarly, in Appendix B of his work, Steven W. Stahler (1983) states that the mass, <math>~M</math>, associated with the equilibrium radius, <math>~R_\mathrm{eq}</math>, of embedded polytropic spheres is,

<math> ~M </math>

<math>~=~</math>

<math> M_\mathrm{SWS} \biggl( \frac{n^3}{4\pi} \biggr)^{1/2} \biggl\{ \theta_n^{(n-3)/2} \xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr\}_{\xi_e} </math>

<math> ~R_\mathrm{eq} </math>

<math>~=~</math>

<math> R_\mathrm{SWS} \biggl( \frac{n}{4\pi} \biggr)^{1/2} \biggl\{ \xi \theta_n^{(n-1)/2} \biggr\}_{\xi_e} </math>

where, from his equations (7) and (B3) we deduce,

<math>M_\mathrm{SWS} = \biggl( \frac{n+1}{nG} \biggr)^{3/2} K_n^{2n/(n+1)} P_\mathrm{e}^{(3-n)/[2(n+1)]} \, ,</math>

<math> R_\mathrm{SWS} = \biggl( \frac{n+1}{nG} \biggr)^{1/2} K_n^{n/(n+1)} P_\mathrm{e}^{(1-n)/[2(n+1)]} \, . </math>

Notice that, via these two normalizations, Stahler chose to express <math>~R_\mathrm{eq}</math> and <math>~M</math> in terms of <math>~K_\mathrm{n}</math> and the applied external pressure, <math>~P_\mathrm{e}</math>.

Reconciliation

Here we demonstrate that Whitworth's and Stahler's presentations are equivalent to one another. We begin by plugging Stahler's definition of <math>~M_\mathrm{SWS}</math> into his expression for <math>~M</math>, then inverting it to obtain an expression for <math>~P_\mathrm{e}</math> in terms of <math>~M</math> and <math>~K_\mathrm{n}</math>.

<math> ~M </math>

<math>~=~</math>

<math> \biggl[ \frac{(n+1)^3}{4\pi G^3} \biggr]^{1/2} K_n^{2n/(n+1)} P_\mathrm{e}^{(3-n)/[2(n+1)]} \biggl\{ \theta_n^{(n-3)/2} \xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr\}_{\xi_e} </math>

<math> \Rightarrow ~~~~~ P_\mathrm{e}^{(3-n)} </math>

<math>~=~</math>

<math> \biggl[ \frac{4\pi G^3}{(n+1)^3} \biggr]^{(n+1)} K_n^{-4n} M^{2(n+1)} \biggl\{ \theta_n^{(n-3)/2} \xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr\}^{-2(n+1)}_{\xi_e} </math>

 

<math>~=~</math>

<math> \biggl[ \frac{4\pi G^3 M^2}{(n+1)^3} \biggr]^{(n+1)} K_n^{-4n} \biggl\{ \theta_n^{(3-n)} \xi^{-4} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}^{(n+1)}_{\xi_e} </math>

Alternatively, plugging Whitworth's definition of <math>~P_\mathrm{rf}</math> into his expression for <math>~P_\mathrm{e}</math> gives,

<math> ~P_\mathrm{e}^{(3-n)} </math>

<math>~=~</math>

<math> 2^{2(5n+1)} \biggl( \frac{\pi}{3^4 \cdot 5^3} \biggr)^{(n+1)} 2^{-8n} \cdot 3^{4(n+1)} \biggl(\frac{5}{n+1} \biggr)^{3(n+1)} [ G^{3} M^{2} ]^{(n+1)} K_n^{-4n} \biggl\{ \theta_n^{(3-n)}\xi^{-4} \biggl|\frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}_{\xi_e}^{(n+1)} </math>

 

<math>~=~</math>

<math> 2^{2(n+1)} \biggl[ \frac{\pi}{(n+1)^3} \biggr]^{(n+1)} [ G^{3} M^{2} ]^{(n+1)} K_n^{-4n} \biggl\{ \theta_n^{(3-n)}\xi^{-4} \biggl|\frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr\}_{\xi_e}^{(n+1)} \, . </math>

So Whitworth's and Stahler's relations for <math>~P_\mathrm{e}(M,K_n)</math> are, indeed, identical. Similarly examining Stahler's expression for the equilibrium radius, we find,

<math> ~R_\mathrm{eq} </math>

<math>~=~</math>

<math> \biggl( \frac{n+1}{4\pi G} \biggr)^{1/2} K_n^{n/(n+1)} \biggl[ \xi \theta_n^{(n-1)/2} \biggr]_{\xi_e} \biggl\{ P_\mathrm{e}^{1/(n+1)} \biggr\}^{(1-n)/2} </math>

 

<math>~=~</math>

<math> \biggl( \frac{n+1}{4\pi G} \biggr)^{1/2} K_n^{n/(n+1)} \biggl[ \xi \theta_n^{(n-1)/2} \biggr]_{\xi_e} \biggl\{ \biggl[ \frac{4\pi G^3 M^2}{(n+1)^3} \biggr] K_n^{-4n/(n+1)} \biggl[ \theta_n^{(3-n)} \xi^{-4} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{-2} \biggr]_{\xi_e} \biggr\}^{(1-n)/[2(3-n)]} </math>

<math> \Rightarrow ~~~~~ R_\mathrm{eq}^{(3-n)} </math>

<math>~=~</math>

<math> \biggl( \frac{n+1}{4\pi G} \biggr)^{(3-n)/2} K_n^{n(3-n)/(n+1)} \xi_e^{3-n} \biggl\{ \biggl[ \frac{4\pi G^3 M^2}{(n+1)^3} \biggr]^{1/2} K_n^{-2n/(n+1)} \biggl[ \xi^{-2} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{-1} \biggr]_{\xi_e} \biggr\}^{(1-n)} </math>

 

<math>~=~</math>

<math> (n+1)^{[(3-n)-3(1-n)]/2} (4\pi)^{[(n-3) +(1-n)]/2} G^{[(n-3)+3(1-n)]/2} [K_n^{(3-n)+2(n-1)}]^{n/(n+1)} \xi_e^{(3-n)+2(n-1)} M^{(1-n)} \biggl| \frac{d\theta}{d\xi} \biggr|^{(n-1)}_{\xi_e} </math>

 

<math>~=~</math>

<math> (n+1)^{n} (4\pi)^{-1} G^{-n} K_n^n M^{(1-n)} \biggl[ \xi^{(n+1)} \biggl| \frac{d\theta_n}{d\xi} \biggr|^{(n-1)}\biggr]_{\xi_e} \, . </math>

And Whitworth's expression becomes,

<math> ~R_\mathrm{eq}^{(3-n)} </math>

<math>~=~</math>

<math> 2^{-2(n+1)} \pi^{-1} 3^{n+1} \cdot 5^{n} K_n^n G^{-n} M^{1-n} \biggl[ \frac{4(n+1)}{5} \biggr]^{n} \biggl(\frac{\xi_e}{3} \biggr)^{(n+1)} \biggl|\frac{d\theta_n}{d\xi} \biggr|^{(n-1)}_{\xi_e} </math>

 

<math>~=~</math>

<math> (n+1)^n (4\pi)^{-1} K_n^n G^{-n} M^{1-n} \xi_e^{(n+1)} \biggl|\frac{d\theta_n}{d\xi} \biggr|^{(n-1)}_{\xi_e} \, . </math>

Hence, Stahler's equilibrium radius, <math>~R_\mathrm{eq}</math>, exactly matches Whitworth's <math>~R_\mathrm{eq}</math>.

Summary

Once the function, <math>~\theta_n(\xi)</math>, and its first derivative with respect to the dimensionless radial coordinate, <math>~d\theta_n/d\xi</math>, are obtained via a solution of the Lane-Emden equation, the equilibrium radius, <math>~R_\mathrm{eq}</math>, and total mass, <math>~M</math>, of a pressure-bounded polytrope can be expressed in terms of Stahler's normalizations as follows:

<math> \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} </math>

<math>~=~</math>

<math> \biggl( \frac{n}{4\pi} \biggr)^{1/2}\biggl[ \xi \theta_n^{(n-1)/2} \biggr]_{\xi_e} \, , </math>

<math> \frac{M}{M_\mathrm{SWS}} </math>

<math>~=~</math>

<math> \biggl( \frac{n^3}{4\pi} \biggr)^{1/2} p_a^{(n-3)/[2(n+1)]} \, , </math>

where,

<math> ~p_a </math>

<math>~\equiv~</math>

<math> \biggl[ \theta^{(n-3)/2}_n \xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr]_{\xi_e}^{2(n+1)/(n-3)} = \theta_n^{(n+1)} \biggl(\xi^2 \biggl| \frac{d\theta_n}{d\xi} \biggr| \biggr)_{\xi_e}^{2(n+1)/(n-3)} \, . </math>

Then, the external pressure, expressed in terms of Whitworth's normalization, is,

<math> \frac{P_\mathrm{e}}{P_\mathrm{rf}} </math>

<math>~=~</math>

<math> ~2^{8n/(n-3)} \biggl[ \frac{(n+1)^3}{3^4\cdot 5^3}\biggr]^{(n+1)/(n-3)} p_a \, ; </math>

and the conversion from Stahler's normalization to Whitworth's normalization of the radius is achieved via the expression,

<math> \frac{R_\mathrm{SWS}}{R_\mathrm{rf}} </math>

<math>~=~</math>

<math> ~\biggl[ \frac{3^{(n+1)}}{2^{(n+3)}} \biggl( \frac{5}{n+1} \biggr)^n \biggr]^{1/(n-3)} \biggl( \frac{\pi}{n} \biggr)^{1/2} p_a^{(1-n)/[2(n+1)]} \, . </math>

Chieze's Presentation

From equations (8), (10), and (68) in Chapter IV of [C67], we can immediately formulate the following expressions for, respectively, <math>~P_e(\tilde\xi), R_\mathrm{eq}(\tilde\xi)</math>, and <math>~M_\mathrm{tot}(\tilde\xi)</math>:

<math>~\frac{P_e}{P_\mathrm{Ch}}</math>

<math>~=</math>

<math>~ {\tilde\theta}^{n+1} \, ,</math>

<math>~\frac{R_\mathrm{eq}}{R_\mathrm{Ch}}</math>

<math>~=</math>

<math>~\biggl[ \frac{n+1}{4\pi} \biggr]^{1 / 2} \tilde\xi</math>

<math>~\frac{M_\mathrm{tot}}{M_\mathrm{Ch}}</math>

<math>~=</math>

<math>~ \biggl[ \frac{(n+1)^3}{4\pi} \biggr]^{1 / 2}(- {\tilde\xi}^2 {\tilde\theta}^') \, , </math>

where,

<math>~P_\mathrm{Ch}</math>

<math>~\equiv</math>

<math>~K\rho_c^{(n+1)/n} \, ,</math>

<math>~R_\mathrm{Ch}</math>

<math>~\equiv</math>

<math>~\biggl[\biggl(\frac{K}{G}\biggr) \rho_c^{1/n-1}\biggr]^{1 / 2} \, ,</math>

<math>~M_\mathrm{Ch}</math>

<math>~\equiv</math>

<math>~\biggl[\biggl(\frac{K}{G}\biggr)^3 \rho_c^{(3-n)/n}\biggr]^{1 / 2} \, .</math>

In this case, the expressions for the physical variable normalizations have been defined in terms of — in addition to <math>~G</math> and/or <math>~K</math> — the equilibrium configuration's central density, <math>~\rho_c</math>, instead of in terms of <math>~M_\mathrm{tot}</math> or <math>~P_e</math>. These are precisely the expressions for, respectively, <math>~P_s(\xi_s)</math>, <math>~R_s(\xi_s)</math>, and <math>~M_s(\xi_s)</math> that appear in the appendix of J. P. Chieze (1987, A&A, 171, 225-232) — see, respectively, his equations (A7), (A5), and (A6). [Note that, for the polytropic systems of interest to us, here — that is, systems having <math>~0 \le n < \infty</math> — Chieze's parameter <math>~\epsilon \equiv \sgn(n+1) = 1</math>.]

Polytropic Configurations with n = 1

Drawing from the earlier discussion of isolated polytropes, we will reference various radial locations within the spherical configuration by the dimensionless radius,

<math> \xi \equiv \frac{r}{a_\mathrm{n=1}} , </math>

where,

<math> a_\mathrm{n=1} \equiv \biggl[\frac{1}{4\pi G}~ \biggl( \frac{H_c}{\rho_c} \biggr)_{n=1}\biggr]^{1/2} = \biggl[\frac{K}{2\pi G} \biggr]^{1/2} \, . </math>

The solution to the Lane-Emden equation for <math>~n = 1</math> is,

<math> ~\theta_1 </math>

<math>~=~</math>

<math> \frac{\sin\xi}{\xi} \, , </math>

hence,

<math> \frac{d\theta_1}{d\xi} </math>

<math>~=~</math>

<math> \frac{\cos\xi}{\xi} - \frac{\sin\xi}{\xi^2} \, . </math>

Review

Again, from the earlier discussion, we can describe the properties of an isolated, spherical <math>~n</math> = 1 polytrope as follows:

  • Mass:
In terms of the central density, <math>\rho_c</math>, and <math>~K_\mathrm{n}</math>, the total mass is,

<math>M = \frac{4}{\pi} \rho_c (\pi a_{n=1})^3 = 4\pi^2 \rho_c \biggl[\frac{K}{2\pi G} \biggr]^{3/2} = \rho_c \biggl[\frac{2\pi K^3}{G^3} \biggr]^{1/2}</math> ;

and, expressed as a function of <math>M</math>, the mass that lies interior to the dimensionless radius <math>\xi</math> is,

<math>\frac{M_\xi}{M} = \frac{1}{\pi} \biggl[ \sin\xi - \xi\cos\xi \biggr] \, ,~~~~~~\mathrm{for}~\pi \ge \xi \ge 0 \, .</math>

Hence,

<math>M_\xi = \rho_c \biggl[\frac{2K^3}{\pi G^3} \biggr]^{1/2} \biggl[ \sin\xi - \xi\cos\xi \biggr] \, .</math>

  • Pressure:
The central pressure of the configuration is,

<math>P_c = \biggl[ \frac{G^3}{2\pi} \rho_c^4 M^2 \biggr]^{1/3} = \biggl[ \frac{G^3}{2\pi} \rho_c^6 \biggl(\frac{2\pi K^3}{G^3} \biggr) \biggr]^{1/3} = K\rho_c^2</math> ;

and, expressed in terms of the central pressure <math>P_c</math>, the variation with radius of the pressure is,

<math>P_\xi= P_c \biggl[ \frac{\sin\xi}{\xi} \biggr]^2</math> .

Hence,

<math>P_\xi= K\rho_c^2 \biggl[ \frac{\sin\xi}{\xi} \biggr]^2</math> .

Extension to Bounded Sphere

Eliminating <math>\rho_c</math> between the last expression for <math>M_\xi</math> and the last expression for <math>P_\xi</math> gives,

<math>P_\xi= \biggl[\frac{\pi}{2} \cdot \frac{G^3 M_\xi^2}{K^2} \biggr] \biggl[ \frac{\sin\xi}{\xi(\sin\xi - \xi \cos\xi )} \biggr]^2</math> .

Now, if we rip off an outer layer of the star down to some dimensionless radius <math>\xi_e < \pi</math>, the interior of the configuration that remains — containing mass <math>M_{\xi_e}</math> — should remain in equilibrium if we impose the appropriate amount of externally applied pressure <math>P_e = P_{\xi_e} </math> at that radius. (This will work only for spherically symmetric configurations, as the gravitation acceleration at any location only depends on the mass contained inside that radius.) If we rescale our solution such that the mass enclosed within <math>\xi_e</math> is the original total mass <math>M</math>, then the pressure that must be imposed by the external medium in which the configuration is embedded is,

<math>P_e= \biggl[\frac{\pi}{2} \cdot \frac{G^3 M^2}{K^2} \biggr] \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2</math> .

The associated equilibrium radius of this pressure-confined configuration is,

<math> R_\mathrm{eq} = \xi_e a_\mathrm{n=1} = \biggl[ \frac{K}{2\pi G} \biggr]^{1/2} \xi_e </math>

Overlap with Whitworth's Presentation

The solid green curve in the two top panels of Figure 1 shows how <math>R_\mathrm{eq}</math> varies with the applied external pressure <math>P_e</math> for this pressure-bounded <math>~n=1</math> model sequence. In the top-right panel, following the lead of Whitworth (1981, MNRAS, 195, 967) — for clarification, read the accompanying ASIDE — these two quantities have been respectively normalized (or, "referenced") to,

<math> R_\mathrm{rf}\biggr|_\mathrm{n=1} \equiv \biggl( \frac{3^2 \cdot 5}{2^4 \pi} \biggr)^{1/2} \biggl(\frac{K}{G}\biggr)^{1/2} ~~~\Rightarrow ~~~ \frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl( \frac{2^3}{3^2 \cdot 5} \biggr)^{1/2} \xi_e \, ; </math>

and,

<math> P_\mathrm{rf}\biggr|_\mathrm{n=1} \equiv \frac{2^6 \pi}{3^4 \cdot 5^3} \biggl(\frac{G^3 M^2}{K^2}\biggr) ~~~\Rightarrow ~~~ \frac{P_e}{P_\mathrm{rf}} = \biggl( \frac{3^4 \cdot 5^3}{2^7} \biggr) \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2 \, . </math>

Note that this pair of mathematical expressions has been recorded to the immediate right of Whitworth's name in our <math>~n=1</math> summary table. In the top-left panel of Figure 1, the solid green curve shows the identical sequence, but plotted as <math>~\log(p_a)</math> versus <math>~log(r_a)</math>, for easier comparison with Horedt's work. The pair of mathematical expressions defining <math>~r_a(\xi_e)</math> and <math>~p_a(\xi_e)</math> has been recorded to the immediate right of Horedt's name in the same summary table.


Figure 1: Equilibrium R-P Diagram — Referred to by Kimura (1981) as an "M1 Sequence"

All of the plots shown in this figure illustrate how the equilibrium radius of a pressure-bounded polytrope varies with the applied external pressure. In the right-hand column, the log-log plots display a normalized <math>~P_e</math> along the horizontal axis and a normalized <math>~R_\mathrm{eq}</math> along the horizontal axis; in the left-hand column, these axes are flipped, and a different normalization is used. One primary intent of all the diagrams is to show that, for polytropic sequences having <math>~n > 3</math> (or, equivalently, sequences having <math>\gamma_g \equiv 1 + 1/n < 4/3),</math> no equilibrium models exist above some limiting external pressure.

To be compared with Horedt (1970)
To be compared with Whitworth (1981)
Horedt (1970) Figure 1
Whitworth (1981) Figure 1b
Horedt (1970) Title Page
Whitworth (1981) Title Page

Bottom Left [reproduction of Figure 1 from Horedt (1970)]: All three displayed sequences — <math>~n=4</math> (<math>~\gamma_g = 1.25</math>), <math>~n=5</math> (<math>~\gamma_g = 1.20</math>), and <math>~n=\infty</math> (<math>~\gamma_g = 1</math>, hence, isothermal) — exhibit an upper limit for the bounding pressure. Each sequence displays two segments — a solid segment and a dashed segment — indicating that, below the maximum allowed value of <math>~P_e</math>, it is possible to construct two (or more) equilibrium configurations; models lying along the solid segment of each displayed curve are expected to be dynamically stable while models lying along the dashed segments are unstable.

Bottom Right [reproduction of Figure 1b from Whitworth (1981)]: Model sequences are shown for five different effective adiabatic indexes — <math>~\gamma_g = 1/3,~ 2/3,~ 1,~ 4/3,</math> and <math>~ 5/3</math> — corresponding, respectively, to polytropic indexes <math>~n = -2/3, -1/3, \infty, ~3/2, </math> and <math>~3</math>. The three sequences having <math>~\gamma_g < 4/3</math> exhibit an upper limit for the bounding pressure. Both the stable (solid) curve segment and the unstable (dashed) curve segment are drawn for the isothermal <math>~(\gamma_g = 1)</math> sequence, which is also displayed (as the <math>~n=\infty</math> sequence) in Horedt's diagram.

Top: Plots that we have generated for direct comparison with Horedt's diagram (left) and with Whitworth's diagram (right). Both plots display only the two sequences that are analytically prescribable: <math>~n=1</math> (<math>~\gamma_g = 2</math>) and <math>~n=5</math> (<math>~\gamma_g = 1.20</math>). Along the <math>~n=1</math> (green) sequence, stable equilibrium models can be constructed for all values of <math>~P_e</math>. Along the <math>~n=5</math> sequence, equilibrium models only exist for values of <math>~P_e</math> less than the critical value, <math>~P_\mathrm{max} = (2^5\cdot 3^9/5^9) P_\mathrm{rf} = (3^{12}/2^{24}) P_\mathrm{Horedt}</math>; below this critical pressure, the sequence has two branches denoted by blue diamonds (stable models) and red squares (unstable models).

Overlap with Stahler's Presentation

We can invert the above expression for <math>~P_e(K,M)</math> to obtain the following expression for <math>~M(K,P_e)</math>:

<math>~M= K \biggl[\frac{2}{\pi} \cdot \frac{P_e}{G^3} \biggr]^{1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr]</math> .

If, following Stahler's lead, we normalize this expression by <math>~M_\mathrm{SWS}</math> (evaluated for <math>~n=1</math>) and we normalize the above expression for <math>~R_\mathrm{eq}</math> by <math>~R_\mathrm{SWS}</math> (evaluated for <math>~n=1</math>), we obtain,

<math> \frac{M}{M_\mathrm{SWS}} </math>

<math>~=~</math>

<math> K \biggl[\frac{2}{\pi} \cdot \frac{P_e}{G^3} \biggr]^{1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr] \biggl[ \biggl( \frac{G}{2} \biggr)^{3/2} K^{-1} P_\mathrm{ex}^{-1/2} \biggr] </math>

 

<math>~=~</math>

<math> (4\pi)^{-1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr] \, , </math>

<math> \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} </math>

<math>~=~</math>

<math> \biggl[ \frac{K}{2\pi G} \biggr]^{1/2} \xi_e \biggl[ \frac{G}{2K} \biggr]^{1/2} = (4\pi)^{-1/2} \xi_e \, . </math>

Figure 2: Equilibrium Mass-Radius Diagram

Stahler (1983) Title Page

Top: A slightly edited reproduction of Figure 17 in association with Appendix B of Stahler (1983, ApJ, 268, 165). Stahler's figure caption reads, in part, "Mass-radius relation for bounded polytropes (schematic). Each curve is labeled by the appropriate value or range" of <math>~n</math> … "As the cloud density increases from unity, all curves leave the origin with the same slope …"


Bottom: Curves depict the exact, analytically derived mass-radius relationship for truncated <math>~n = 1</math> (purple squares) and <math>~n = 5</math> (blue diamonds) polytropes that are embedded in an external medium of pressure <math>~P_e</math>; the relevant mathematical expressions are presented to the immediate right of Stahler's name in, respectively, our <math>~n=1</math> summary table and our <math>~n=5</math> summary table. As the dimensionless truncation radius, <math>~\xi_e</math>, increases steadily from zero, both curves exhibit very similar behavior up to <math>~M_n \equiv M/M_\mathrm{SWS} \approx 0.5</math>; thereafter the normalized mass and normalized radius continue to steadily increase along the <math>~n = 1</math> sequence, but the <math>~n = 5</math> sequence eventually bends back on itself, returning to the origin as <math>~\xi_e \rightarrow \infty</math>.


Comparison: The monotonic <math>P-R</math> behavior of the analytically derived solution for <math>~n</math> = 1 <math>(\gamma_g = 2)</math>, shown above, is consistent with the behavior of the numerically derived solutions presented by Whitworth for slightly lower values of <math>\gamma_g</math> = 5/3 and 4/3. The analytically derived solution for <math>~n</math> = 5 <math>(\gamma_g = 6/5)</math> shows that, above some limiting pressure, no equilibrium configuration exists; this is consistent with the behavior of the numerically derived solutions presented by Whitworth for all values of <math>\gamma_g < 4/3 \, .</math>

Stahler (1983) Figure 17 (edited)
To be compared with Stahler (1983)

Tabular Summary (n=1)

Table 1:  Properties of <math>~n=1</math> Polytropes Embedded in an External Medium of Pressure <math>~P_e</math>
(and, accordingly, truncated at radius <math>~\xi_e</math>)

<math> ~\theta_1 = \frac{\sin\xi_e}{\xi_e} </math>

        and        

<math> ~\frac{d\theta_1}{d\xi} \biggr|_{\xi_e} = \frac{\cos\xi_e}{\xi_e} - \frac{\sin\xi_e}{\xi_e^2} </math>

Horedt (1970)
for
fixed <math>~(M,K_n)</math>

<math> ~r_a = \frac{R_\mathrm{eq}}{R_\mathrm{Horedt}} = \xi_e </math>

<math> ~p_a = \frac{P_e}{P_\mathrm{Horedt}} = \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2 </math>

Whitworth (1981)
for
fixed <math>~(M,K_n)</math>

<math> \frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl( \frac{2^3}{3^2 \cdot 5} \biggr)^{1/2} \xi_e </math>

<math> \frac{P_e}{P_\mathrm{rf}} = \biggl( \frac{3^4 \cdot 5^3}{2^7} \biggr) \biggl[ \frac{\sin\xi_e}{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )} \biggr]^2 </math>

Stahler (1983)
for
fixed <math>~(P_e,K_n)</math>

<math> \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} = (4\pi)^{-1/2} \xi_e </math>

<math> \frac{M}{M_\mathrm{SWS}} = (4\pi)^{-1/2} \biggl[ \frac{\xi_e(\sin\xi_e - \xi_e \cos\xi_e )}{\sin\xi_e} \biggr] </math>

NOTE: None of the analytic expressions for the dimensionless radius, pressure, or mass presented in this table explicitly appear in the referenced articles by Horedt, by Whitworth, or by Stahler but, as is discussed fully above, they are straightforwardly derivable from the more general relations that appear in these papers.

Polytropic Configurations with n = 5

Drawing from the earlier discussion of isolated polytropes, we will reference various radial locations within a spherical <math>~n</math> = 5 polytrope by the dimensionless radius,

<math> \xi \equiv \frac{r}{a_\mathrm{n=5}} , </math>

where,

<math> a_{n=5} = \biggr[ \frac{(n+1)K}{4\pi G} \rho_c^{(1/n - 1)} \biggr]^{1/2}_{n=5} = \biggr[ \frac{3K}{2\pi G} \biggr]^{1/2} \rho_c^{-2/5} \, . </math>

The solution to the Lane-Emden equation for <math>~n = 5</math> is,

<math> ~\theta_5 </math>

<math>~=~</math>

<math> \biggl(1+\frac{\xi^2}{3} \biggr)^{-1/2} \, , </math>

hence,

<math> \frac{d\theta_5}{d\xi} </math>

<math>~=~</math>

<math> - \frac{\xi}{3}\biggl(1+\frac{\xi^2}{3} \biggr)^{-3/2} \, . </math>

Review

Again, from the earlier discussion, we can describe the properties of an isolated, spherical <math>~n</math> = 5 polytrope as follows:

  • Mass:
In terms of the central density, <math>\rho_c</math>, and <math>~K_\mathrm{n}</math>, the total mass is,

<math>M = \biggr[ \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr]^{1/2} \rho_c^{-1/5} </math> ;

and, expressed as a function of <math>M</math>, the mass that lies interior to the dimensionless radius <math>\xi</math> is,

<math> \frac{M_\xi}{M} = \xi^3 (3 + \xi^2)^{-3/2} \, . </math>

Hence,

<math> M_\xi = \biggr[ \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr]^{1/2} \rho_c^{-1/5} \biggl[ \xi^3 (3 + \xi^2)^{-3/2} \biggr] \, . </math>

  • Pressure:
The central pressure of the configuration is,

<math> P_c = \biggr[ \frac{\pi M^2 G^3}{2\cdot 3^4} \biggr]^{1/3} \rho_c^{4/3} = \biggr[ \frac{\pi G^3}{2\cdot 3^4} \biggr( \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr) \rho_c^{-2/5}\biggr]^{1/3} \rho_c^{4/3} = K\rho_c^{6/5} </math> ;

and, expressed in terms of the central pressure <math>P_c</math>, the variation with radius of the pressure is,

<math>P_\xi= P_c \biggl[ 1 + \frac{1}{3}\xi^2 \biggr]^{-3}</math> .

Hence,

<math> P_\xi = K \rho_c^{6/5} \biggl[ 1 + \frac{1}{3}\xi^2 \biggr]^{-3} = 3^3K \rho_c^{6/5} [ 3 + \xi^2 ]^{-3} </math> .

Extension to Bounded Sphere

Eliminating <math>\rho_c</math> between the last expression for <math>M_\xi</math> and the last expression for <math>P_\xi</math> gives,

<math>~P_\xi</math> <math>=</math>

<math> 3^3K [ 3 + \xi^2 ]^{-3} \biggr[ \frac{2\cdot 3^4 K^3}{\pi G^3} \biggr]^{3} M_\xi^{-6} \biggl[ \xi^3 (3 + \xi^2)^{-3/2} \biggr]^6 </math>

  <math>=</math>

<math> \biggl( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 M_\xi^{6} G^9} \biggr) \frac{\xi^{18}}{(3 + \xi^2)^{12}} \, . </math>

Now, if we rip off an outer layer of the star down to some dimensionless radius <math>\xi_e < \infty</math>, the interior of the configuration that remains — containing mass <math>M_{\xi_e}</math> — should remain in equilibrium if we impose the appropriate amount of externally applied pressure <math>P_e = P_{\xi_e} </math> at that radius. (This will work only for spherically symmetric configurations, as the gravitation acceleration at any location only depends on the mass contained inside that radius.) If we rescale our solution such that the mass enclosed within <math>\xi_e</math> is the original total mass <math>M</math>, then the pressure that must be imposed by the external medium in which the configuration is embedded is,

<math>P_e= \biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 M^{6} G^9} \biggr) \frac{\xi_e^{18}}{(3 + \xi_e^2)^{12}} </math> .

The associated equilibrium radius of this pressure-confined configuration is,

<math> R_\mathrm{eq} = \xi_e a_\mathrm{n=5} = \biggl[ \frac{3K}{2\pi G} \biggr]^{1/2} \rho_c^{-2/5} \xi_e = \biggl[ \frac{\pi M^4 G^5}{2^3 \cdot 3^7 K^5} \biggr]^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5} \, . </math>

Overlap with Whitworth's Presentation

The curve labeled <math>~n=5</math> in the top two panels of Figure 1 shows how <math>R_\mathrm{eq}</math> varies with the applied external pressure <math>P_e</math>; as shown, the curve has two segments — configurations that are stable (blue diamonds) and configurations that are unstable (red squares). Following the lead of Whitworth (1981, MNRAS, 195, 967) — for clarification, read the accompanying ASIDE — these two quantities have been respectively normalized (or, "referenced") to,

<math> R_\mathrm{rf}\biggr|_\mathrm{n=5} \equiv \frac{2^6}{3^3} \biggl( \frac{\pi}{5^5} \biggr)^{1/2} \biggl[ \frac{G^5 M^4}{K^5} \biggr]^{1/2} ~~~\Rightarrow ~~~ \frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl( \frac{5^5}{2^{15}\cdot 3} \biggr)^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5} \, ; </math>

and,

<math> P_\mathrm{rf}\biggr|_\mathrm{n=5} \equiv \frac{3^{12} 5^9}{2^{26} \pi^3} \biggl( \frac{K^{10}}{G^9 M^6} \biggr) ~~~\Rightarrow ~~~ \frac{P_e}{P_\mathrm{rf}} = \biggl( \frac{2^{29}\cdot 3^{3} }{5^9} \biggr) \frac{\xi_e^{18}}{(3 + \xi_e^2)^{12}} \, . </math>

We see that this <math>~n=5</math> model sequence bends back on itself. That is to say, for this polytropic index there is an externally applied pressure above which no equilibrium configuration exists. This limiting pressure arises along the curve where,

<math>\frac{dP_e}{dR_\mathrm{eq}} = \biggl( \frac{dP_e}{d\xi_e} \biggr) \biggl( \frac{dR_\mathrm{eq}}{d\xi_e} \biggr)^{-1} = 0 \, .</math>

Evaluation of this expression shows that the limiting pressure occurs precisely at <math>\xi_e = 3</math>, that is,

<math> \biggl( \frac{P_e}{P_\mathrm{rf}} \biggr)_\mathrm{max} = \biggl( \frac{2^{29}\cdot 3^{3} }{5^9} \biggr) \frac{3^{18}}{12^{12}} = \frac{2^5 \cdot 3^9}{5^9} \, , </math>

and the radius of this limiting configuration is,

<math> \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{rf}} \biggr) = \biggl( \frac{5^5}{2^{15}\cdot 3} \biggr)^{1/2} \frac{12^3}{3^5} = \biggl( \frac{5^5}{2^3 \cdot 3^5} \biggr)^{1/2} \, . </math>

On the log-log plot displayed in the top-right panel of Figure 1, the location of this special point is <math>[ \log(P_e/P_\mathrm{rf}) , \log(R_\mathrm{eq}/R_\mathrm{rf}) ] \approx [ -0.49149, +0.10308 ] \, .</math>

We note as well that a conversion from Whitworth's normalizations to the normalizations adopted by Horedt produce the following coordinates for the limiting model configuration:

<math> ~p_a|_\mathrm{max} </math>

<math>~=~</math>

<math> ~\frac{3^{12}}{2^{24}} \, , </math>

and, at this bounding pressure, the model has an equilibrium radius,

<math> ~r_a </math>

<math>~=~</math>

<math> \frac{2^6}{3^3} \, . </math>

Overlap with Stahler's Presentation

We can invert the above expression for <math>~P_e(K,M)</math> to obtain the following expression for <math>~M(K,P_e)</math>:

<math>M^{6}= \biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 P_e G^9} \biggr) \frac{\xi_e^{18}}{(3 + \xi_e^2)^{12}} </math> .

If, following Stahler's lead, we normalize this expression by <math>~M_\mathrm{SWS}</math> (evaluated for <math>~n=5</math>) and we normalize the above expression for <math>~R_\mathrm{eq}</math> by <math>~R_\mathrm{SWS}</math> (evaluated for <math>~n=5</math>), we obtain,

<math> \frac{M}{M_\mathrm{SWS}} </math>

<math>~=~</math>

<math> \biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 P_e G^9} \biggr)^{1/6} \frac{\xi_e^{3}}{(3 + \xi_e^2)^{2}} \biggl[ \biggl( \frac{2\cdot 3}{5G} \biggr)^{3/2} K^{5/3} P_\mathrm{ex}^{-1/6} \biggr]^{-1} </math>

 

<math>~=~</math>

<math> \biggr( \frac{3^{2} \cdot 5^3 }{4\pi } \biggr)^{1/2} \frac{\xi_e^{3}}{(3 + \xi_e^2)^{2}} \, , </math>

<math> \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} </math>

<math>~=~</math>

<math> \biggl[ \frac{\pi M^4 G^5}{2^3 \cdot 3^7 K^5} \biggr]^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5} \biggl[ \biggl( \frac{2\cdot 3}{5G} \biggr)^{1/2} K^{5/6} P_\mathrm{ex}^{-1/3} \biggr]^{-1} </math>

 

<math>~=~</math>

<math> \biggl[ \biggr( \frac{2^3\cdot 3^{15} K^{10}}{\pi^3 P_e G^9} \biggr)^{1/3} \frac{\xi_e^{6}}{(3 + \xi_e^2)^{4}} \biggr] \biggl[ \frac{\pi G^5}{2^3 \cdot 3^7 K^5} \biggr]^{1/2} \frac{(3+\xi_e^2)^3}{\xi_e^5} \biggl( \frac{5G}{2\cdot 3} \biggr)^{1/2} \biggl[ K^{-5/6} P_\mathrm{ex}^{1/3} \biggr] </math>

 

<math>~=~</math>

<math> \biggr( \frac{3^{2} \cdot 5}{2^2 \pi } \biggr)^{1/2} \frac{\xi_e}{(3 + \xi_e^2)} \, . </math>

This set of parametric relations that relate the mass of the truncated configuration to its radius via the parameter, <math>~\xi_e</math>, has been recorded to the immediate right of Stahler's name in our <math>~n=5</math> summary table, below.

Stahler points out (see his equation B13) that, for this particular pressure-bounded polytropic sequence, <math>~\xi_e</math> can be eliminated between the expressions to obtain the following direct algebraic relationship between <math>~M</math> and <math>~R_\mathrm{eq}</math>:

<math> \biggl( \frac{M}{M_\mathrm{SWS}} \biggr)^2 - 5 \biggl( \frac{M}{M_\mathrm{SWS}} \biggr)\biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr) + \frac{20\pi}{3} \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)^4 </math>

<math>~=~</math>

<math> ~0 \, . </math>

Viewed as a quadratic equation in the mass, the roots of this expression give,

<math> \frac{M}{M_\mathrm{SWS}} </math>

<math>~=~</math>

<math> \frac{5}{2} \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr) \biggl\{ 1 \pm \biggl[ 1 - \frac{16\pi}{15} \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)^2 \biggr]^{1/2} \biggr\} \, . </math>

[CORRECTION: Changed factor inside square root from <math>~16\pi/3</math> to <math>~16\pi/15</math> on 24 December 2014.] We have used this expression to generate the complete <math>~n=5</math> sequence shown here in the top panel of Figure 2 — the solid green segment of the curve shows the negative root and the solid red segment of the curve was generated using the positive root.


ASIDE: In his Appendix B, Stahler (1983) claims that the quadratic equation relating <math>~M</math> directly to <math>~R_\mathrm{eq}</math> (his equation B13) can be obtained by analytically integrating the first-order ordinary differential equation presented as his equation B10. I don't think that this is possible without knowing ahead of time how <math>~M</math> relates to <math>~R_\mathrm{eq}</math> through the above-derived parametric relations in <math>~\xi_e</math>.

[29 September 2014 by J. E. Tohline] Now that (I think) I've finished deriving the properly defined virial equilibrium condition for embedded polytropes and have reconciled that equilibrium expression with Horedt's corresponding specification of the equilibrium radius and surface-pressure, it's time to revisit the concern that was expressed in this "ASIDE" regarding the mass-radius relationship for embedded, <math>~n=5</math> polytropes presented by Stahler.

Tabular Summary (n=5)

Table 2:  Properties of <math>~n=5</math> Polytropes Embedded in an External Medium of Pressure <math>~P_e</math>
(and, accordingly, truncated at radius <math>~\xi_e</math>)

<math> ~\theta_5 = \biggl( 1 + \frac{\xi_e^2}{3} \biggr)^{-1/2} </math>

        and        

<math> ~\frac{d\theta_5}{d\xi} \biggr|_{\xi_e} = - \frac{\xi_e}{3} \biggl( 1 + \frac{\xi_e^2}{3} \biggr)^{-3/2} </math>

Horedt (1970)
for
fixed <math>~(M,K_n)</math>

<math> ~r_a = \frac{R_\mathrm{eq}}{R_\mathrm{Horedt}} = \biggl\{ 3 \biggl[ \frac{(\xi_e^2/3)^5}{(1+\xi_e^2/3)^{6}} \biggr] \biggr\}^{-1/2} </math>

<math> ~p_a = \frac{P_e}{P_\mathrm{Horedt}} = 3^3 \biggl[ \frac{(\xi_e^2/3)^3}{(1+\xi_e^2/3)^{4}} \biggr]^3 </math>

Whitworth (1981)
for
fixed <math>~(M,K_n)</math>

<math> \frac{R_\mathrm{eq}}{R_\mathrm{rf}} = \biggl\{ \frac{2^{15}}{5^5} \biggl[ \frac{(\xi_e^2/3)^5}{(1+\xi_e^2/3)^{6}} \biggr] \biggr\}^{-1/2} </math>

<math> \frac{P_e}{P_\mathrm{rf}} = \frac{2^{29}}{5^9} \biggl[ \frac{(\xi_e^2/3)^3}{(1+\xi_e^2/3)^{4}} \biggr]^3 </math>

Stahler (1983)
for
fixed <math>~(P_e,K_n)</math>

<math> \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} = \biggl\{ \frac{3\cdot 5}{2^2 \pi} \biggl[ \frac{\xi_e^2/3}{(1+\xi_e^2/3)^{2}} \biggr] \biggr\}^{1/2} </math>

<math> \frac{M}{M_\mathrm{SWS}} = \biggl[ \biggl( \frac{3 \cdot 5^3}{2^2\pi} \biggr) \frac{(\xi_e^2/3)^3}{(1+\xi_e^2/3)^{4}} \biggr]^{1/2} </math>

<math> \biggl( \frac{M}{M_\mathrm{SWS}} \biggr)^2 - 5 \biggl( \frac{M}{M_\mathrm{SWS}} \biggr)\biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr) + \frac{2^2 \cdot 5 \pi}{3} \biggl( \frac{R_\mathrm{eq}}{R_\mathrm{SWS}} \biggr)^4 = 0 </math>

NOTE: None of the analytic expressions for the dimensionless radius, pressure, or mass presented in this table explicitly appear in the referenced articles by Horedt, by Whitworth, or by Stahler but, as is discussed fully above, they are straightforwardly derivable from the more general relations that appear in these papers. The final polynomial relating the dimensionless mass to the dimensionless radius does explicitly appear as equation (B13) in Stahler (1983).

Additional discussion of Stahler's analytic mass-radius relation is presented in an accompanying chapter.

Additional, Numerically Constructed Polytropic Configurations

As has been detailed in an accompanying chapter, using numerical techniques we have solved the Lane-Emden equation, and thereby discerned the internal structural profiles, for polytropes having a wide variety of polytropic indexes. The righthand panel of Figure 3 presents a diagram in which the mass-radius "sequences" corresponding to eight different polytropic indexes have been drawn.


file = Dropbox/WorkFolder/Wiki edits/EmbeddedPolytropes/CombinedSequences.xlsx --- worksheet = EqSeqCombined2
Figure 3:   Mass-Radius Behavior of Various Polytropic Sequences
Stahler (1983) Figure 17 (edited)

Combined DFB Sequences

Turning Points

Limiting Pressure Along M1 Sequence

As is illustrated in the figures presented above, when an equilibrium sequence is constructed for any bounded (pressure-truncated) configuration having <math>~n > 3</math>, the sequence exhibits multiple "turning points." For example, when moving along the R-P sequence displayed in Figure 1 for <math>~n=5</math> configurations, the external pressure monotonically climbs to a maximum value, <math>~P_\mathrm{max}</math>, then "turns around" and steadily decreases thereafter. Horedt (1970) and Kimura (1981b) separately derived an expression that pinpoints the location of the <math>~P_\mathrm{max}</math> turning point along an R-P sequence — Kimura refers to this as an "M1 sequence" because the configuration's mass is held fixed while the external pressure and the system's corresponding equilibrium radius is varied. The turning point is located along the sequence at the point where,

<math> \frac{d P_e}{d R_\mathrm{eq}} \biggr|_M </math>

<math>~=~</math>

<math> 0\, , </math>

or, just as well, where,

<math> \frac{d \ln P_e}{d\ln R_\mathrm{eq}} \biggr|_M </math>

<math>~=~</math>

<math> 0\, . </math>

In what follows, we examine the expressions derived by both authors in order to show that they are identical to one another as well as to re-express the result in a form that conforms to our own adopted notation.

Horedt's Derivation

Appreciating that Horedt's notation for the surface pressure of an equilibrium configuration — which equals the applied external pressure <math>~P_e</math> — is <math>~\tilde{p}</math>, and his notation for <math>~R_\mathrm{eq}</math> is <math>~\tilde{r}</math>, the requisite expression from Horedt's paper [see also equation (13) in Viala & Horedt (1974)] is the one displayed in the following boxed image:

Excerpt from Horedt (1970)

Viala & Horedt (1974) Expressions

where,

Viala & Horedt (1974) Expressions

That is, from Horedt's work we have,

<math>~\frac{dP_e}{dR_\mathrm{eq}}\biggr|_M ~~\rightarrow ~~ \frac{d\tilde{p}}{d \tilde{r}}</math>

<math>~\sim</math>

<math>~\frac{(3-n)(n+1)(\tilde\theta^')^2 + (2n+2)\tilde\theta^{n+1}}{(1-n)\tilde\xi f^' + (3-n)(n+1)(\tilde\theta^')^2} \, .</math>

Let's independently derive this relation, starting from Horedt's equilibrium expressions for <math>~\tilde{r}</math> and <math>~\tilde{p}</math>, as summarized above. (For purposes of simplification, we will for the most part drop the tilde notation.)

<math>~ \frac{1}{R_\mathrm{Horedt}} \cdot \frac{d\tilde{r}}{d \tilde\xi}</math>

<math>~=</math>

<math>~ \frac{d}{d\xi}\biggl[ \tilde\xi ( -\tilde\xi^2 \tilde\theta' )^{(1-n)/(n-3)}\biggr] </math>

 

<math>~=</math>

<math>~ (-\xi^2\theta^')^{(1-n)/(n-3)}\biggl[ 1 +\frac{(1-n)}{(n-3)} \cdot \xi (-\xi^2\theta^')^{-1} (-\xi^2\theta^')^' \biggr] </math>

 

<math>~=</math>

<math>~\frac{1}{(n-3)(n+1)}\cdot (-\xi^2\theta^')^{(1-n)/(n-3)} \biggl[ (n-3)(n+1)

+(n-1)\cdot (\theta^')^{-2} \xi f^'

\biggr] </math>

 

<math>~=</math>

<math>~\frac{(-\xi^2\theta^')^{(1-n)/(n-3)} }{(3-n)(n+1)(\theta^')^{2}} \biggl[ (3-n)(n+1)(\theta^')^{2} +(1-n) \xi f^' \biggr] \, ; </math>

<math>~ \frac{1}{P_\mathrm{Horedt}} \cdot \frac{d\tilde{p}}{d \tilde\xi}</math>

<math>~=</math>

<math>~ \frac{d}{d\xi}\biggl[ \tilde\theta^{n+1}( -\tilde\xi^2 \tilde\theta' )^{2(n+1)/(n-3)} \biggr] </math>

 

<math>~=</math>

<math>~ ( -\tilde\xi^2 \tilde\theta' )^{2(n+1)/(n-3)} \biggl[ f^' + f \cdot \frac{2(n+1)}{(n-3)}( -\tilde\xi^2 \tilde\theta' )^{-1} ( -\tilde\xi^2 \tilde\theta' )^' \biggr] </math>

 

<math>~=</math>

<math>~ ( -\tilde\xi^2 \tilde\theta' )^{2(n+1)/(n-3)} \biggl[ f^' - \frac{2(n+1)}{(n-3)(n+1)} \cdot \frac{f\cdot f^'}{(\theta^')^2} \biggr] </math>

 

<math>~=</math>

<math>~ \frac{f^' ( -\tilde\xi^2 \tilde\theta' )^{(3n+1)/(n-3)} ( -\tilde\xi^2 \tilde\theta' )^{(1-n)/(n-3)} }{(3-n)(n+1)(\theta^')^2} \biggl[ (3-n)(n+1)(\theta^')^2 + 2(n+1) \theta^{n+1} \biggr] \, . </math>

The ratio of these two expressions gives,

<math>~\frac{R_\mathrm{Horedt}}{P_\mathrm{Horedt}} \cdot \frac{dP_e}{dR_\mathrm{eq}}\biggr|_M </math>

<math>~=</math>

<math>~f^' ( -\tilde\xi^2 \tilde\theta' )^{(3n+1)/(n-3)} \biggl\{ \frac{(3-n)(n+1)(\theta^')^2 + 2(n+1) \theta^{n+1}}{(3-n)(n+1)(\theta^')^{2} +(1-n) \xi f^' } \biggr\} \, , </math>

completing our task, as the term inside the curly braces exactly matches the equation excerpt from Horedt's work, as displayed above.

Kimura's Derivation

Appreciating that Kimura uses the subscript "1," rather than a tilde, to identify equilibrium parameter values, the requisite expression is equation (22) from Kimura's "Paper II," as displayed in the following boxed image:

Excerpts (edited) from Kimura (1981b)

Kimura (1981b) Expressions

where,

Kimura (1981b) Expressions

Drawing on the additional parameter and variable definitions provided in our discussion of Kimura's presentation, above, we can rewrite this key expression as,

<math>~\frac{R_\mathrm{eq}}{P_e} \cdot \frac{dP_e}{dR_\mathrm{eq}}\biggr|_M ~~\rightarrow ~~ \frac{d\ln{p_1}}{d \ln{r_1}}</math>

<math>~=</math>

<math>~ \frac{v_G \cdot h_G}{k_G} \, ,</math>

where,

<math>~v_G</math>

<math>~=</math>

<math>~\frac{2}{[1-2(n+1)^{-1}]} =\frac{ 2(n+1)}{n-1} \, , </math>

<math>~u_G</math>

<math>~=</math>

<math>~(3-1)-\biggl[\frac{1}{1-2(n+1)^{-1}} \biggr] = 2-\frac{(n+1)}{(n-1)} = \frac{(n-3)}{(n-1)} \, , </math>

<math>~h_G</math>

<math>~=</math>

<math>~ \frac{1}{u_G} \biggl[ \frac{\zeta \theta^n}{\phi^'} \biggr]_1 - \frac{1}{v_G} \biggl[ \frac{\zeta \phi^'}{\theta} \biggr]_1 = \frac{(n-1)}{(n-3)} \biggl[ \frac{\tilde\xi \tilde\theta^n}{-\tilde\theta^'} \biggr] - \frac{(n-1)}{2(n+1)} \biggl[ \frac{(n+1)\tilde\xi (-\tilde\theta^')}{\tilde\theta} \biggr] </math>

 

<math>~=</math>

<math>~ \frac{(n-1)\tilde\xi}{2(n+1)(n-3)\tilde\theta (-\tilde\theta^')} \biggl\{ 2(n+1) \tilde\theta^{n+1} + (3-n) (n+1) (-\tilde\theta^')^2 \biggr\} \, , </math>

<math>~k_G</math>

<math>~=</math>

<math>~1- \frac{1}{u_G} \biggl[ \frac{\zeta \theta^n}{\phi^'} \biggr]_1 =1- \frac{(n-1)}{(n-3)} \biggl[ \frac{\tilde\xi \tilde\theta^n}{-\tilde\theta^'} \biggr] = \frac{1}{ (n-3) (-\tilde\theta^') } \biggl\{ (n-3)(- \tilde\theta^') - (n-1) \tilde\xi \tilde\theta^n \biggr\} </math>

 

<math>~=</math>

<math>~ \frac{1}{ (n+1)(n-3) (-\tilde\theta^')^2 } \biggl\{ (n-3)(n+1) (-\tilde\theta^')^2 - (n-1)\tilde\xi [(n+1) \tilde\theta^n (-\tilde\theta^')] \biggr\} \, . </math>

 

<math>~=</math>

<math>~ \frac{-1}{ (n+1)(n-3) (-\tilde\theta^')^2 } \biggl\{ (3-n)(n+1) (-\tilde\theta^')^2 + (1-n)\tilde\xi [(n+1) \tilde\theta^n (\tilde\theta^')] \biggr\} \, . </math>

Hence, from Kimura's work we find,

<math>~\frac{R_\mathrm{eq}}{P_e} \cdot \frac{dP_e}{dR_\mathrm{eq}}\biggr|_M </math>

<math>~=</math>

<math>~ \frac{(n+1)\tilde\xi \tilde\theta^'}{\tilde\theta} \biggl\{ \frac{2(n+1) \tilde\theta^{n+1} + (3-n) (n+1) (\tilde\theta^')^2}{(3-n)(n+1) (\tilde\theta^')^2 + (1-n)\tilde\xi [(n+1) \tilde\theta^n \tilde\theta^'] } \biggr\} \, . </math>

Appreciating that <math>~f^' = [(n+1)\tilde\theta^n \tilde\theta^']</math>, we see that the expression inside the curly braces here matches exactly the expression inside the curly braces that was obtained through Horedt's derivation, as it should! The prefactor is different in the two expressions only because Kimura's result is for a logarithmic derivative whereas Horedt's derivation is not; the ratio of the two prefactors is, simply, the ratio,

<math>~\frac{R_\mathrm{eq}/R_\mathrm{Horedt}}{P_e/P_\mathrm{Horedt}} </math>

<math>~=</math>

<math>~\frac{\tilde\xi}{\tilde\theta_n^{n+1}}\cdot ( -\tilde\xi^2 \tilde\theta' )^{[(1-n)-2(n+1)]/(n-3)} </math>

 

<math>~=</math>

<math>~\frac{\tilde\xi}{\tilde\theta_n^{n+1}}\cdot ( -\tilde\xi^2 \tilde\theta' )^{-(3n+1)/(n-3)} \, . </math>

In a separate discussion, specifically focused on the <math>~n=5</math> mass-radius relationship, we show how Kimura's analysis of turning points can be usefully applied.

Location of Pressure Limit

Now we can identify the location along the M1 sequence where the turning point set by <math>~P_\mathrm{max}</math> occurs by setting the numerator of this expression equal to zero, specifically,

<math>~2(n+1) \tilde\theta^{n+1} + (3-n) (n+1) (\tilde\theta^')^2 </math>

<math>~=</math>

<math>~ 0 \, . </math>

This means that the equilibrium model that sits at the <math>~P_\mathrm{max}</math> turning point will have,

<math>~\frac{\tilde\theta^{n+1}}{(\tilde\theta^')^2} </math>

<math>~=</math>

<math>~ \frac{(n-3)}{2} \, . </math>

Other Limits

In a similar fashion, Kimura (1981b) derived mathematical expressions that identify the location of other turning points along equilibrium sequences of bounded polytropic configurations. An M1 sequence — as displayed, for example, in the set of P-R diagrams shown in Figure 1, above — exhibits not only an "extremal of p1" but also an "extremal of r1." As we have just reviewed, the first of these is identified by setting <math>~(d\ln p_1/d\ln r_1)_{M} = 0</math> or, using Kimura's more compact terminology, the first occurs at a location that satisfies the condition,

<math>h_G = 0 \, ,</math>       that is, where …       <math>~\tilde\theta^{n+1} (\tilde\theta^')^{-2} = (n-3)/2 \, .</math>

Similarly, Kimura points out that an "extremal in r1" along an M1 sequence occurs at a location that satisfies the condition,

<math>k_G = 0 \, ,</math>       that is, where …       <math>~\tilde\xi \tilde\theta^{n} (-\tilde\theta^')^{-1} = (n-3)/(n-1) \, .</math>

As is illustrated by the plots presented in Figure 2, above, turning points also arise in the mass-radius relationship of bounded polytropic configurations having <math>~n > 3</math>. These are identified by Kimura as "p1 sequences" because the external pressure is held fixed while the system's mass and corresponding equilibrium radius is varied. In §5 of his "Paper II," Kimura points out that the same two conditions — namely, <math>~h_G = 0</math> and <math>~k_G = 0</math> — also identify the location of extrema in M1 along, respectively, p1 sequences and r1 sequences.


We can also identify extrema in r1 along p1 sequences by setting <math>~(\dot{p}_1/p_1) = 0</math> in Kimura's equation (17), then substituting the resulting expression for the function <math>~Z</math>, namely,

<math>~Z = v_1 \, ,</math>

into his equations (15) and (16). The ratio of these two resulting expressions gives,

<math>~\frac{d\ln M_1}{d \ln r_1}\biggr|_{p_1}</math>

<math>~=</math>

<math>~ \frac{u_1 -(u_G/v_G)v_1}{1 - v_1/v_G} = [u_1 v_G - u_G v_1][v_G - v_1]^{-1} </math>

 

<math>~=</math>

<math>~ \biggl[\frac{2(n+1)}{(n-1)} \cdot \frac{\xi \theta^n}{(-\theta^')} - \frac{(n-3)}{(n-1)} \cdot \frac{(n+1)\xi (-\theta^')}{\theta} \biggr] \biggl[\frac{2(n+1)}{(n-1)} - \frac{(n+1)\xi (-\theta^')}{\theta} \biggr]^{-1} </math>

 

<math>~=</math>

<math>~ \frac{\xi }{(-\theta^')} \biggl[ \frac{2 \theta^{n+1} - (n-3) (-\theta^')^2 }{2\theta - (n-1)\xi (-\theta^') } \biggr] </math>

As has just been reviewed, the condition <math>~h_G=0</math> results from setting the numerator of this expression equal to zero and identifies extrema in M1 along p1 sequences. In addition, now, we can identify the condition for extrema in r1 along p1 sequences by setting the denominator to zero. The condition is,

<math>~\frac{\xi (-\theta^')}{\theta} = \frac{2}{(n-1)} \, .</math>

Some Tabulated Values

Table 3:   Turning-Point Locations along M-R Sequences of Pressure-Truncated Polytropes
n Maximum Radius Maximum Mass
<math>~\tilde\xi</math> <math>~\tilde\theta</math> <math>~\biggl|\frac{d\theta}{d\xi}\biggr|_\tilde\xi</math> <math>~\frac{(n-1)}{2}\biggl[ \frac{\xi}{\theta} \biggl|\frac{d\theta}{d\xi}\biggr|~\biggr]_\tilde\xi</math> <math>~\frac{R}{R_\mathrm{SWS}}</math> <math>~\frac{M}{M_\mathrm{SWS}}</math> <math>~\tilde\xi</math> <math>~\tilde\theta</math> <math>~\biggl|\frac{d\theta}{d\xi}\biggr|_\tilde\xi</math> <math>~\frac{(n-3)}{2}\biggl[ \frac{1}{\theta^{n+1}} \biggl(\frac{d\theta}{d\xi}\biggr)^2 \biggr]_\tilde\xi</math> <math>~\frac{R}{R_\mathrm{SWS}}</math> <math>~\frac{M}{M_\mathrm{SWS}}</math>
3 2.172 0.5387 0.2496 1.006 0.5717 1.726 6.89684862 0.0 -0.04242976 -- 0.0 2.9583456
3.05 2.162 0.5437 0.2479 1.010 0.5704 1.715 5.034 0.1152 0.07842 0.973 0.2707 2.829
3.5 2.050 0.5930 0.2340 1.011 0.5630 1.594 3.910 0.2788 0.1126 0.994 0.4180 2.311
5 <math>~\sqrt{3}</math> <math>~\frac{1}{\sqrt{2}}</math> <math>~\frac{1}{\sqrt{24}}</math> <math>~1</math> <math>~\biggl( \frac{3\cdot 5}{2^4\pi}\biggr)^{1 / 2}</math> <math>~\biggl( \frac{3\cdot 5^3}{2^6\pi}\biggr)^{1 / 2}</math> <math>~3</math> <math>~\frac{1}{2}</math> <math>~\frac{1}{8}</math> <math>~1</math> <math>~\biggl( \frac{3^2\cdot 5}{2^6\pi}\biggr)^{1 / 2}</math> <math>~\biggl( \frac{3^4\cdot 5^3}{2^{10}\pi}\biggr)^{1 / 2}</math>
6 1.6 0.7510 0.1884 1.003 0.5404 1.301 2.7 0.5811 0.1221 0.999 0.4802 1.635

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Recommended citation:   Tohline, Joel E. (2021), The Structure, Stability, & Dynamics of Self-Gravitating Fluids, a (MediaWiki-based) Vistrails.org publication, https://www.vistrails.org/index.php/User:Tohline/citation