User:Tohline/SSC/FreeEnergy/PolytropesEmbedded
Free-Energy of Truncated Polytropes
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In this case, the Gibbs-like free energy is given by the sum of three separate energies,
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<math>~\mathfrak{G}</math> |
<math>~=</math> |
<math>~W_\mathrm{grav} + \mathfrak{S}_\mathrm{therm} + P_eV</math> |
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<math>~=</math> |
<math>~- \biggl[\frac{3}{5}\cdot \frac{\tilde{\mathfrak{f}}_W}{\tilde{\mathfrak{f}}_M^2} \biggr] \frac{GM^2}{R} - \biggl[\biggl(\frac{3}{4\pi}\biggr)^{1/n} \frac{\tilde{\mathfrak{f}}_A}{\tilde{\mathfrak{\mathfrak{f}}}_M^{(n+1)/n}} \biggr] \frac{KM^{(n+1)/n}}{R^{3/n}} + \frac{4\pi}{3} \cdot P_e R^3 \, ,</math> |
where, as derived elsewhere,
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Structural Form Factors for Pressure-Truncated Polytropes <math>~(n \ne 5)</math> |
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As we have shown separately, for the singular case of <math>~n = 5</math>,
where, <math>~\ell \equiv \tilde\xi/\sqrt{3} </math> |
In general, then, the warped free-energy surface drapes across a four-dimensional parameter "plane" such that,
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<math>~\mathfrak{G}</math> |
<math>~=</math> |
<math>~\mathfrak{G}(R, K, M, P_e) \, .</math> |
In order to effectively visualize the structure of this free-energy surface, we will reduce the parameter space from four to two, in two separate ways: First, we will hold constant the parameter pair, <math>~(K,M)</math>; adopting Kimura's (1981b) nomenclature, we will refer to the resulting function, <math>~\mathfrak{G}_{K,M}(R,P_e)</math>, as an "M1 Free-Energy Surface" because the mass is being held constant. Second, we will hold constant the parameter pair, <math>~(K,P_e)</math>, and examine the resulting "P1 Free-Energy Surface," <math>~\mathfrak{G}_{K,P_e}(R,M)</math>.
The M1 Free-Energy Surface
It is useful to rewrite the free-energy function in terms of dimensionless parameters. Here we need to pick normalizations for energy, radius, and pressure that are expressed in terms of the gravitational constant, <math>~G</math>, and the two fixed parameters, <math>~K</math> and <math>~M</math>. We have chosen to use,
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<math>~R_\mathrm{norm}</math> |
<math>~\equiv</math> |
<math>~\biggl[ \biggl( \frac{G}{K} \biggr)^n M_\mathrm{tot}^{n-1} \biggr]^{1/(n-3)} \, ,</math> |
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<math>~P_\mathrm{norm}</math> |
<math>~\equiv</math> |
<math>~\biggl[ \frac{K^{4n}}{G^{3(n+1)} M_\mathrm{tot}^{2(n+1)}} \biggr]^{1/(n-3)} \, ,</math> |
which, as is detailed in an accompanying discussion, are similar but not identical to the normalizations used by Horedt (1970) and by Whitworth (1981). The self-consistent energy normalization is,
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<math>~E_\mathrm{norm}</math> |
<math>~\equiv</math> |
<math>~P_\mathrm{norm} R^3_\mathrm{norm} \, .</math> |
As we have demonstrated elsewhere, after implementing these normalizations, the expression that describes the M1 Free-Energy surface is,
<math> \mathfrak{G}_{K,M}^* \equiv \frac{\mathfrak{G}_{K,M}}{E_\mathrm{norm}} = -3A\biggl(\frac{R}{R_\mathrm{norm}}\biggr)^{-1} -~ nB \biggl(\frac{R}{R_\mathrm{norm}}\biggr)^{-3/n} +~ \biggl( \frac{4\pi}{3} \biggr) \frac{P_e}{P_\mathrm{norm}} \biggl(\frac{R}{R_\mathrm{norm}}\biggr)^3 \, , </math>
where the constants,
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<math>~A</math> |
<math>~\equiv</math> |
<math>\frac{1}{5} \cdot \frac{\tilde{\mathfrak{f}}_W}{\tilde{\mathfrak{f}}_M^2} \, ,</math> |
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<math>~B</math> |
<math>~\equiv</math> |
<math>~ \biggl(\frac{4\pi}{3} \biggr)^{-1/n} \frac{\tilde{\mathfrak{f}}_A}{\tilde{\mathfrak{f}}_M^{(n+1)/n}} \, . </math> |
Given the polytropic index, <math>~n</math>, we expect to obtain a different M1 free-energy surface for each choice of the dimensionless truncation radius, <math>~\tilde\xi</math>; this choice will imply corresponding values for <math>~\tilde\theta</math> and <math>~\tilde\theta^'</math> and, hence also, corresponding (constant) values of the coefficients, <math>~A</math> and <math>~B</math>.
See Also
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© 2014 - 2021 by Joel E. Tohline |