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Whitworth's (1981) Isothermal Free-Energy Surface
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Power-Law Density Distributions (structure)

LSU Structure still.gif

Here we begin with the same second-order, one-dimensional ODE that governs the structure of polytropic spheres, namely, the

Lane-Emden Equation
<math>\frac{1}{\xi^2} \frac{d}{d\xi}\biggl( \xi^2 \frac{d\Theta_H}{d\xi} \biggr) = - \Theta_H^n</math> ,

and examine whether or not this governing relation can be satisfied by a power-law enthalpy distribution of the form,

<math> \Theta_H = A \xi^{-\alpha} , </math>

where <math>A</math> and <math>\alpha</math> are assumed to be constants. We note, up front, that such a solution will not satisfy the boundary conditions that are imposed on polytropic spheres. But the simplistic form of a power-law solution can nevertheless sometimes be instructive.


Plugging the power-law expression for the dimensionless enthalpy into both sides of the Lane-Emden equation gives,


-\alpha (1 -\alpha) A \xi^{-(2 +\alpha)}  = - A^n \xi^{-\alpha n} .


Hence, the power-law enthalpy distribution works as long as,

<math> \alpha = \frac{2}{n-1} ~~~~~~\mathrm{and}~~~~~~ A = [\alpha (1 -\alpha)]^{1/(n-1)} = \biggl[ \frac{2(n-3)}{(n-1)^2} \biggr]^{1/(n-1)}. </math>

This means that hydrostatic balance can be established at all radial positions within a spherically symmetric configuration for power-law density distributions of the form,

<math> \frac{\rho}{\rho_c} = \biggl[ \frac{2(n-3)}{(n-1)^2} \biggr]^{n/(n-1)} \xi^{- 2n/(n-1)}. </math>

(Note that, in this case, the subscript c should not represent the central conditions but, rather, conditions at some characteristic radial position within the configuration.)


It looks like the derived solution makes some physical sense only for polytropic indices <math> n > 3</math>. For <math>n=4</math>, the relevant power-law density distribution is,

<math> \frac{\rho}{\rho_c} = \biggl[ \frac{2}{9} \biggr]^{4/3} \xi^{- 8/3}. </math>

For <math>n=(3+\epsilon)</math> and <math>\epsilon \ll 1</math>,

<math> \frac{\rho}{\rho_c} \approx \biggl[ \frac{\epsilon}{2} \biggr] \xi^{- 3}. </math>

For <math>n \gg 1</math>,

<math> \frac{\rho}{\rho_c} \approx \biggl[ \frac{2}{n} \biggr] \xi^{- 2}. </math>

Hence, for polytropic indices in the range <math>\infty > n > 3</math>, the relevant power-law density distribution lies between <math> \rho \propto \xi^{-2}</math> and <math> \rho \propto \xi^{-3}</math>.

Related Wikipedia Discussions

Whitworth's (1981) Isothermal Free-Energy Surface

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