User:Tohline/SSC/Structure/BiPolytropes/Analytic1 5

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BiPolytrope with <math>n_c = 1</math> and <math>n_e=5</math>

Whitworth's (1981) Isothermal Free-Energy Surface
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Here we construct a bipolytrope in which the core has an <math>~n_c=1</math> polytropic index and the envelope has an <math>~n_e=5</math> polytropic index. As in the case of our separately discussed, "mirror image" bipolytropes having <math>~(n_c, n_e) = (5, 1)</math>, this system is particularly interesting because the entire structure can be described by closed-form, analytic expressions. [On 12 April 2015, J. E. Tohline wrote: I became aware of the published discussions of this system by Murphy — and especially the work of Murphy & Fiedler (1985) — (see itemization of additional key references, below) in March of 2015 after searching the internet for previous analyses of radial oscillations in polytropes and, then, reading through Horedt's (2004) §2.8.1 discussion of composite polytropes.]

Key References

Steps 2 & 3

Based on the discussion presented elsewhere of the structure of an isolated <math>n=1</math> polytrope, the core of this bipolytrope will have the following properties:

<math> \theta(\xi) = \frac{\sin\xi}{\xi} ~~~~\Rightarrow ~~~~ \theta_i = \frac{\sin\xi_i}{\xi_i} ; </math>

<math> \frac{d\theta}{d\xi} = \frac{\cos\xi}{\xi}- \frac{\sin\xi}{\xi^2} ~~~~\Rightarrow ~~~~ \biggl(\frac{d\theta}{d\xi}\biggr)_i = \frac{\cos\xi_i}{\xi_i}- \frac{\sin\xi_i}{\xi_i^2} \, . </math>

The first zero of the function <math>~\theta(\xi)</math> and, hence, the surface of the corresponding isolated <math>~n=1</math> polytrope is located at <math>~\xi_s = \pi</math>. Hence, the interface between the core and the envelope can be positioned anywhere within the range, <math>~0 < \xi_i < \pi</math>.

Step 4: Throughout the core (<math>0 \le \xi \le \xi_i</math>)

Specify: <math>~K_c</math> and <math>~\rho_0 ~\Rightarrow</math>

 

<math>~\rho</math>

<math>~=</math>

<math>\rho_0 \theta^{n_c}</math>

<math>~=</math>

<math>~\rho_0 \biggl( \frac{\sin\xi}{\xi} \biggr)</math>

<math>~P</math>

<math>~=</math>

<math>~K_c \rho_0^{1+1/n_c} \theta^{n_c + 1}</math>

<math>~=</math>

<math>~K_c \rho_0^{2} \biggl( \frac{\sin\xi}{\xi}\biggr)^{2}</math>

<math>~r</math>

<math>~=</math>

<math>~\biggl[ \frac{(n_c + 1)K_c}{4\pi G} \biggr]^{1/2} \rho_0^{(1-n_c)/(2n_c)} \xi</math>

<math>~=</math>

<math>~\biggl[ \frac{K_c}{2\pi G} \biggr]^{1/2} \xi</math>

<math>~M_r</math>

<math>~=</math>

<math>~4\pi \biggl[ \frac{(n_c + 1)K_c}{4\pi G} \biggr]^{3/2} \rho_0^{(3-n_c)/(2n_c)} \biggl(-\xi^2 \frac{d\theta}{d\xi} \biggr)</math>

<math>~=</math>

<math>~4\pi \biggl[ \frac{K_c}{2\pi G} \biggr]^{3/2} \rho_0 \biggl[\sin\xi - \xi \cos\xi \biggr]</math>

Step 5: Interface Conditions

 

Setting <math>~n_c=1</math>, <math>~n_e=5</math>, and <math>~\phi_i = 1 ~~~~\Rightarrow</math>

<math>~\frac{\rho_e}{\rho_0}</math>

<math>~=</math>

<math>~\biggl( \frac{\mu_e}{\mu_c} \biggr) \theta^{n_c}_i \phi_i^{-n_e}</math>

<math>~=</math>

<math>~\biggl( \frac{\mu_e}{\mu_c} \biggr) \theta_i </math>

<math>~\biggl( \frac{K_e}{K_c} \biggr) </math>

<math>~=</math>

<math>~\rho_0^{1/n_c - 1/n_e}\biggl( \frac{\mu_e}{\mu_c} \biggr)^{-(1+1/n_e)} \theta^{1 - n_c/n_e}_i</math>

<math>~=</math>

<math>~\biggl[\rho_0^{4}\biggl( \frac{\mu_e}{\mu_c} \biggr)^{-6} \theta^{4}_i\biggr]^{1/5}</math>

<math>~\frac{\eta_i}{\xi_i}</math>

<math>~=</math>

<math>~\biggl[ \frac{n_c + 1}{n_e+1} \biggr]^{1/2} \biggl( \frac{\mu_e}{\mu_c}\biggr) \theta_i^{(n_c-1)/2} \phi_i^{(1-n_e)/2}</math>

<math>~=</math>

<math>~\biggl( \frac{1}{3} \biggr)^{1/2} \biggl( \frac{\mu_e}{\mu_c}\biggr) </math>

<math>~\biggl( \frac{d\phi}{d\eta} \biggr)_i</math>

<math>~=</math>

<math>~\biggl[ \frac{n_c + 1}{n_e + 1} \biggr]^{1/2} \theta_i^{- (n_c + 1)/2} \phi_i^{(n_e+1)/2} \biggl( \frac{d\theta}{d\xi} \biggr)_i</math>

<math>~=</math>

<math>~\biggl( \frac{1}{3} \biggr)^{1/2} \theta_i^{- 1} \biggl( \frac{d\theta}{d\xi} \biggr)_i</math>

Related Discussions


Whitworth's (1981) Isothermal Free-Energy Surface

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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