User:Tohline/SphericallySymmetricConfigurations/Virial

Virial Equilibrium of Spherically Symmetric Configurations

Free Energy Expression

Review

As has been introduced elsewhere in a more general context, associated with any isolated, self-gravitating, gaseous configuration we can identify a total Gibbs-like free energy, <math>\mathfrak{G}</math>, given by the sum of the relevant contributions to the total energy of the configuration,

<math> \mathfrak{G} = W_\mathrm{grav} + \mathfrak{S}_\mathrm{therm} + T_\mathrm{kin} + P_e V + \cdots </math>

Here, we have explicitly included the gravitational potential energy, <math>~W_\mathrm{grav}</math>, the ordered kinetic energy, <math>~T_\mathrm{kin}</math>, a term that accounts for surface effects if the configuration of volume <math>~V</math> is embedded in an external medium of pressure <math>~P_e,</math> and <math>~\mathfrak{S}_\mathrm{therm}</math>, the reservoir of thermodynamic energy that is available to perform work as the system expands or contracts. A mathematical expression encapsulating the physical definition of each of these energy terms, in full three-dimensional generality, can be found in our introductory discussion of the scalar virial theorem and the free-energy function.

Expressions for Various Energy Terms

We begin, here, by deriving each of the terms in the Gibbs-like free-energy expression as appropriate for spherically symmetric systems. In deriving each term, we keep in mind two issues: First, for a given size system, <math>~R,</math> a determination of each term's total contribution to the free energy generally will involve integration through the entire volume of the configuration, effectively "summing up" the differential mass in each radial shell,

<math> dm = \rho(\vec{x}) d^3x = 4\pi \rho(r) r^2 dr \, , </math>

weighted by some specific energy expression. Second, each term must be formulated in such a way that it is clear how the energy contribution depends on the overall system size, <math>~R.</math>

Volume Integrals

We note, first, that the mass enclosed within each interior radius, <math>~r</math>, is

<math>~M_r(r) = \int\limits_V dm</math>

<math>~=</math>

<math>~ \int_0^r 4\pi r^2 \rho dr \, ,</math>

hence, the total mass is,

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

<math>~=</math>

<math>~ \int_0^R 4\pi r^2 \rho dr \, .</math>

Confinement by External Pressure: For spherically symmetric configurations, the energy term due to confinement by an external pressure can be expressed, simply, in terms of the configuration's radius, <math>~R</math>, as,

<math>~P_e V</math>

<math>~=</math>

<math>~\frac{4\pi}{3} P_e R^3 \, .</math>

Gravitational Potential Energy: From our discussion of the scalar virial theorem — see, specifically, the reference to Equation (18), on p. 18 of EFE — the gravitational potential energy is given by the expression,

<math> W_\mathrm{grav} = - \int\limits_V \rho x_i \frac{\partial\Phi}{\partial x_i} d^3 x = - \int\limits_V \vec{r} \cdot \nabla\Phi dm = - \int_0^R \biggl( r \frac{d\Phi}{dr} \biggr) dm \, . </math>

For spherically symmetric systems, the

Poisson Equation

<math>\nabla^2 \Phi = 4\pi G \rho</math>

becomes,

<math>~\frac{1}{r^2} \frac{d}{dr} \biggl( r^2 \frac{d\Phi}{dr} \biggr) </math>

<math>~=</math>

<math>~4\pi G \rho(r) \, , </math>

which implies,

<math>~r^2 \frac{d\Phi}{dr} </math>

<math>~=</math>

<math>~\int_0^r 4\pi G \rho(r) r^2 dr = GM_r(r) \, .</math>

Hence — see, also, p. 64, Equation (12) of Chandrasekhar [C67] — the desired expression for the gravitational potential energy is,

<math>~W_\mathrm{grav}</math>

<math>~=</math>

<math>~ - \int_0^R \biggl( \frac{GM_r}{r} \biggr) dm = - \int_0^R \frac{G}{r}\biggl[\int_0^r 4\pi r^2 \rho dr \biggr] 4\pi r^2 \rho dr \, .</math>

Also, as pointed out by Chandrasekhar [C67] — see p. 64, Equation (16) — it may sometimes prove advantageous to recognize that, if a spherically symmetric system is in hydrostatic balance, an alternate expression for the total gravitational potential energy is, <math>~W_\mathrm{grav}</math> <math>~=</math> <math>~ + \frac{1}{2} \int_0^R \Phi(r) dm \, .</math>

Rotational Kinetic Energy: We will also consider a system that is rotating with a specified simple angular velocity profile, <math>~\dot\varphi(\varpi)</math>, in which case, from our discussion of the scalar virial theorem — see, specifically, the reference to Equation (8), on p. 16 of EFE — the (ordered) kinetic energy,

<math>~T_\mathrm{kin}</math>

<math>~=</math>

<math>~ \frac{1}{2} \int\limits_V \rho |\vec{v} |^2 d^3x = \frac{1}{2} \int\limits_V |\vec{v} |^2 dm \, ,</math>

is entirely rotational kinetic energy, specifically,

<math>~T_\mathrm{kin} = T_\mathrm{rot}</math>

<math>~=</math>

<math>~ \frac{1}{2} \int\int\int \dot\varphi^2 \varpi^2 dm = \frac{1}{2} \int_0^R \dot\varphi^2 \varpi^2 \int_{-\sqrt{R^2 - \varpi^2}}^{\sqrt{R^2 - \varpi^2}} \rho(r(\varpi,z)) 2\pi \varpi d\varpi dz\, .</math>

Reservoir of Thermodynamic Energy: As has been explained in our introductory discussion of the Gibbs-like free energy, formulation of an expression for the reservoir of thermodynamic energy, <math>~\mathfrak{S}_\mathrm{therm}</math>, depends on whether the system is expected to evolve adiabatically or isothermally. For isothermal systems,

<math> \mathfrak{S}_\mathrm{therm} ~~\rightarrow ~~\mathfrak{S}_I = + \int\limits_V c_s^2 \ln \biggl(\frac{\rho}{\rho_0}\biggr) dm = c_s^2 \int_0^R \ln \biggl(\frac{\rho}{\rho_0}\biggr) 4\pi r^2 \rho dr \, , </math>

where, <math>~c_s</math> is the isothermal sound speed and <math>~\rho_0</math> is a (as yet unspecified) reference mass density; while, for adiabatic systems,

<math> \mathfrak{S}_\mathrm{therm} ~~\rightarrow ~~ \mathfrak{S}_A = + \int\limits_V \frac{1}{({\gamma_g}-1)} \biggl( \frac{P}{\rho} \biggr) dm = \frac{1}{({\gamma_g}-1)} \int_0^R 4\pi r^2 P dr

\, ,</math>

where, <math>~P(r)</math> is the system's pressure distribution and <math>~\gamma_g</math> is the specified adiabatic index.

Idealized Configuration

In the idealized situation of a configuration that has uniform density, <math>~\rho_c</math>, has uniform pressure, <math>~P_c</math>, and is uniformly rotating with angular velocity, <math>~\dot\varphi_c</math>, the mass interior to each radius and the total mass are, respectively,

<math>~M_r</math>	<math>~=</math>	<math>~4\pi \rho_c R^3 \int_0^x x^2 dx = \frac{4\pi}{3} \rho_c R^3 x^3 \, ,</math>
<math>~M_\mathrm{tot} </math>	<math>~=</math>	<math>~M_r (x=1) = \frac{4\pi}{3} \rho_c R^3 \, ,</math>

and evaluation of the various energy integrals yields,

<math>~W_\mathrm{grav}</math>	<math>~=</math>	<math>~ - \biggl( \frac{2^4 \pi^2}{3} \biggr) G\rho_c^2 R^5 \int_0^1 x^4 dx = - \frac{3}{5} \biggl( \frac{GM_\mathrm{tot}^2}{R} \biggr) \, ,</math>
<math>~T_\mathrm{kin}</math>	<math>~=</math>	<math>~ 2\pi R^5 \rho_c \dot\varphi^2_c \int_0^1 w^3 dw \int_0^{\sqrt{1 - w^2}} d\zeta = 2\pi R^5 \rho_c \dot\varphi^2_c \int_0^1 w^3 (1-w^2)^{1/2} dw </math>
	<math>~=</math>	<math>~ 2\pi R^5 \rho_c \dot\varphi^2_c \biggl[ -\frac{1}{15} (1-w^2)^{3/2} (3w^2 +2) \biggr]_0^1 = \frac{4\pi}{15} R^5 \rho_c \dot\varphi^2_c = \frac{1}{5} M_\mathrm{tot} R^2 \dot\varphi^2_c \, ,</math>
<math>~\mathfrak{S}_I</math>	<math>~=</math>	<math>~ c_s^2 \ln \biggl(\frac{\rho_c}{\rho_0}\biggr) 4\pi R^3 \rho_c \int_0^1 x^2 dx = c_s^2 M_\mathrm{tot} \ln \biggl(\frac{\rho_c}{\rho_0}\biggr) \, ,</math>
<math>~\mathfrak{S}_A</math>	<math>~=</math>	<math>~\frac{4\pi R^3 P_c}{({\gamma_g}-1)} \int_0^1 x^2 dx = \frac{M_\mathrm{tot}}{({\gamma_g}-1)} \frac{P_c}{\rho_c} \, ,</math>

where the various dimensionless integration variables are, <math>~x \equiv (r/R)</math>, <math>~\zeta \equiv (z/R)</math>, and <math>~w \equiv (\varpi/R)</math>.

Structural Form Factors

Keeping in mind the expressions that arise in the case of our just-defined, idealized configuration, in more realistic cases we generally will write the expression for the total mass as,

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

<math>~=</math>

<math>~ \frac{4\pi}{3} R^3 \rho_c \cdot \mathfrak{f}_M ~~~~~ \biggl(\Rightarrow ~ \mathfrak{f}_M = \frac{\bar\rho}{\rho_c} \biggr) \, ,</math>

and we generally will write each energy term as follows:

<math>~W_\mathrm{grav}</math>	<math>~=</math>	<math>~ - \frac{3}{5} \biggl( \frac{GM_\mathrm{tot}^2}{R} \biggr) \cdot \frac{\mathfrak{f}_W}{\mathfrak{f}_M^2} \, ,</math>
<math>~T_\mathrm{kin}</math>	<math>~=</math>	<math>~ \frac{1}{2} \biggl( \frac{2}{5} M_\mathrm{tot} R^2 \biggr) \dot\varphi^2_c \cdot \frac{\mathfrak{f}_T }{\mathfrak{f}_M} \, ,</math>
<math>~\mathfrak{S}_I</math>	<math>~=</math>	<math>~ M_\mathrm{tot} c_s^2 \biggl[\ln\biggl( \frac{\rho_c}{\rho_0} \biggr) + \frac{\mathfrak{f}_I }{\mathfrak{f}_M} \biggl] \, ,</math>
<math>~\mathfrak{S}_A</math>	<math>~=</math>	<math>~ \frac{M_\mathrm{tot}}{({\gamma_g}-1)} \frac{P_c}{\rho_c} \cdot \frac{\mathfrak{f}_A}{\mathfrak{f}_M} \, ,</math>

where the dimensionless form factors, <math>~\mathfrak{f}_i</math> — each usually of order unity — are,

<math>~\mathfrak{f}_M = \frac{\bar\rho}{\rho_c}</math>	<math>~=</math>	<math>~ 3\int_0^1 \biggl[ \frac{\rho(x)}{\rho_c}\biggr] x^2 dx \, ,</math>
<math>~\mathfrak{f}_W</math>	<math>~\equiv</math>	<math>~ 3\cdot 5 \int_0^1 \biggl\{ \int_0^x \biggl[ \frac{\rho(x)}{\rho_c}\biggr] x^2 dx \biggr\} \biggl[ \frac{\rho(x)}{\rho_c}\biggr] x dx\, ,</math>
<math>~\mathfrak{f}_T</math>	<math>~\equiv</math>	<math>~ \frac{15}{2} \int_0^1 \biggl[ \frac{\dot\varphi(w)}{\dot\varphi_c} \biggr]^2 w^3 dw \int_0^{\sqrt{1 - w^2}} \biggl[ \frac{\rho(w,\zeta)}{\rho_c} \biggr] d\zeta\, ,</math>
<math>~\mathfrak{f}_I</math>	<math>~\equiv</math>	<math>~ 3\int_0^1 \biggl[ \frac{\rho(x)}{\rho_c}\biggr] \ln \biggl[ \frac{\rho(x)}{\rho_c}\biggr] x^2 dx \, ,</math>
<math>~\mathfrak{f}_A</math>	<math>~\equiv</math>	<math>~ 3\int_0^1 \biggl[ \frac{P(x)}{P_c}\biggr] x^2 dx \, .</math>

In each case, the "idealized" energy expression is retrieved if/when the relevant form factor, <math>~\mathfrak{f}_i</math>, is set to unity.

Normalizations

It is now appropriate for us to define some characteristic scales against which various physical parameters can be normalized — and, hence, their relative significance can be specified or measured — as the free energy of various systems is examined. As the system size is varied in search of extrema in the free energy, we generally will hold constant the total system mass and the specific entropy of each fluid element. (When isothermal rather than adiabatic variations are considered, the sound speed rather than the specific entropy will be held constant.) Hence, following the lead of both Horedt (1970) and Whitworth (1981), we will express the various characteristic scales in terms of the constants, <math>~G, M_\mathrm{tot},</math> and the polytropic constant, <math>~K.</math> Specifically, we will normalize all length scales, pressures, energies, and mass densities by, respectively,

Adopted Normalizations
<math>~R_\mathrm{norm}</math> <math>~\equiv</math> <math>~\biggl[ \biggl( \frac{G}{K} \biggr) M_\mathrm{tot}^{2-\gamma} \biggr]^{1/(4-3\gamma)} </math> <math>~P_\mathrm{norm}</math> <math>~\equiv</math> <math>~\biggl[ \frac{K^4}{G^{3\gamma} M_\mathrm{tot}^{2\gamma}} \biggr]^{1/(4-3\gamma)} </math> <math>~E_\mathrm{norm}</math> <math>~\equiv</math> <math>~\biggl[ KG^{3(1-\gamma)}M_\mathrm{tot}^{6-5\gamma} \biggr]^{1/(4-3\gamma)} </math> <math>~\rho_\mathrm{norm}</math> <math>~\equiv</math> <math>~\frac{3M_\mathrm{tot}}{4\pi R_\mathrm{norm}^3} = \frac{3}{4\pi} \biggl[ \frac{K^3}{G^3 M_\mathrm{tot}^2} \biggr]^{1/(4-3\gamma )} </math>
Note that, given the above definitions, the following relations hold: <math>~P_\mathrm{norm} R_\mathrm{norm}^4 = E_\mathrm{norm} R_\mathrm{norm} = G M_\mathrm{tot}^2</math>

(Appropriate normalizations for isothermal systems can be obtained by setting <math>~\gamma = 1</math> and by replacing <math>~K</math> with <math>c_\mathrm{s}^2</math> in all four of the above expressions.) As is detailed in a related discussion, our definitions of <math>~R_\mathrm{norm}</math> and <math>~P_\mathrm{norm}</math> are close, but not identical, to the scalings adopted by Horedt (1970) and by Whitworth (1981). The following relations can be used to switch from our normalizations to theirs:

Hoerdt's (1970) Normalization <math>~\biggl( \frac{R_\mathrm{Hoerdt}}{R_\mathrm{norm}} \biggr)^{4-3\gamma}</math> <math>~=</math> <math>~ \frac{(\gamma-1)}{\gamma} \biggl( 4\pi \biggr)^{\gamma-1}</math> <math>~\biggl( \frac{P_\mathrm{Hoerdt}}{P_\mathrm{norm}} \biggr)^{4-3\gamma}</math> <math>~=</math> <math>~ \biggl[\frac{\gamma}{(\gamma-1)} \biggr]^{3\gamma} \biggl( \frac{1}{4\pi} \biggr)^{\gamma}</math>

Whitworth's (1981) Normalization <math>~\biggl( \frac{R_\mathrm{rf}}{R_\mathrm{norm}} \biggr)^{4-3\gamma}</math> <math>~=</math> <math>~ \frac{1}{5\pi} \biggl( \frac{4\pi}{3} \biggr)^\gamma</math> <math>~\biggl( \frac{P_\mathrm{rf}}{P_\mathrm{norm}} \biggr)^{4-3\gamma}</math> <math>~=</math> <math>~ 2^{-2(4+\gamma)} \biggl( \frac{3^4 \cdot 5^3}{\pi} \biggr)^\gamma</math>

It is also worth noting how the length-scale normalization that we are adopting here relates to the characteristic length scale,

<math>~a_n \equiv \biggl[ \frac{1}{4\pi G} \biggl( \frac{H_c}{\rho_c} \biggr) \biggr]^{1/2} \, ,</math>

that has classically been adopted in the context of the Lane-Emden equation, the solution of which provides a detailed description of the internal structure of spherical polytropes for a wide range of values of the polytropic index, <math>~n</math>. Recognizing that, via the polytropic equation of state, the pressure, density, and enthalpy of every element of fluid are related to one another via the expressions,

<math>~H\rho = (n+1)P</math>     … and …      <math>P = K\rho^{1+1/n} \, ,</math>

the specific enthalpy at the center of a polytropic sphere, <math>~H_c/\rho_c</math>, can be rewritten in terms of <math>~K</math> and <math>~\rho_c</math> to give,

<math>~a_n = \biggl[ \frac{(n+1)K}{4\pi G} \rho_c^{(1/n) -1} \biggr]^{1/2} \, ,</math>

which is the definition of this classical length scale introduced by Chandrasekhar (1967) [C67] (see, specifically, his equation 10 on p. 87). Switching from <math>~n</math> to the associated adiabatic exponent via the relation, <math>~\gamma = 1+1/n ~~~\Rightarrow~~~ n = 1/(\gamma-1)</math>, we see that,

<math>~\biggl( \frac{a_n}{R_\mathrm{norm}} \biggr)^2</math>	<math>~=</math>	<math>~\biggl( \frac{\gamma}{\gamma-1} \biggr) \frac{K \rho_c^{(\gamma-2)}}{4\pi G} \cdot \frac{1}{R_\mathrm{norm}^2}</math>
	<math>~=</math>	<math>~\frac{1}{4\pi}\biggl( \frac{\gamma}{\gamma-1} \biggr) \frac{K }{G} \biggl( \frac{\rho_c}{\bar\rho} \biggr)^{(\gamma-2)} \biggl( \frac{3M_\mathrm{tot}}{4\pi R_\mathrm{eq}^3} \biggr)^{(\gamma-2)} \cdot \frac{1}{R_\mathrm{norm}^2} </math>
	<math>~=</math>	<math>~\frac{1}{4\pi} \biggl( \frac{\gamma}{\gamma-1} \biggr) \biggl( \frac{3}{4\pi } \cdot \frac{\rho_c}{\bar\rho} \biggr)^{\gamma-2} \biggl[ \frac{K M_\mathrm{tot}^{\gamma-2} }{G} \biggr] \biggl( \frac{R_\mathrm{norm}}{R_\mathrm{eq}} \biggr)^{3{(\gamma-2)}} \cdot \frac{1}{R_\mathrm{norm}^{3\gamma-4}} </math>
	<math>~=</math>	<math>~\frac{1}{4\pi} \biggl( \frac{\gamma}{\gamma-1} \biggr) \biggl( \frac{4\pi }{3} \cdot \mathfrak{f}_M \biggr)^{2-\gamma} \chi_\mathrm{eq}^{6-3\gamma} \biggl[ \frac{K M_\mathrm{tot}^{\gamma-2} }{G} \biggr] \cdot \biggl[ \biggl( \frac{G}{K} \biggr) M_\mathrm{tot}^{2-\gamma} \biggr] </math>
	<math>~=</math>	<math>~\frac{1}{4\pi} \biggl( \frac{\gamma}{\gamma-1} \biggr) \biggl( \frac{4\pi }{3} \cdot \mathfrak{f}_M \biggr)^{2-\gamma} \chi_\mathrm{eq}^{6-3\gamma} \, . </math>

Notice that, written in this manner, the scale length, <math>~a_n</math>, cannot actually be determined unless the normalized equilibrium radius, <math>~\chi_\mathrm{eq}</math>, is known. We will encounter analogous situations whenever the free energy function is used to identify the physical parameters that define equilibrium configurations — key attributes of a system that should be held fixed as the system size (or some other order parameter) is varied cannot actually be evaluated until an extremum in the free energy is identified and the corresponding value of <math>~\chi_\mathrm{eq}</math> is known. Because solutions of the Lane-Emden equation directly provide detailed force-balance models of polytropic spheres, Chandrasekhar (1967) [C67] did not encounter this issue. As we have discussed elsewhere, the equilibrium radius of a polytropic sphere is identified as the radial location,

<math>~\xi_1 = \frac{R_\mathrm{eq}}{a_n} \, ,</math>

at which the Lane-Emden function, <math>~\Theta_H(\xi)</math>, first goes to zero. Bypassing the free-energy analysis and using knowledge of <math>~\xi_1</math> to identify the equilibrium radius — specifically, setting,

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

<math>~=</math>

<math>~\frac{R_\mathrm{eq}}{R_\mathrm{norm}} = \xi_1 \biggl(\frac{a_n}{R_\mathrm{norm}} \biggr) \, ,</math>

we can extend the above analysis to obtain,

<math>~\biggl( \frac{a_n}{R_\mathrm{norm}} \biggr)^2</math>	<math>~=</math>	<math>~\frac{1}{4\pi} \biggl( \frac{\gamma}{\gamma-1} \biggr) \biggl( \frac{4\pi }{3} \cdot \mathfrak{f}_M \biggr)^{2-\gamma} \biggl[ \xi_1 \biggl(\frac{a_n}{R_\mathrm{norm}} \biggr) \biggr]^{6-3\gamma} </math>
<math>\Rightarrow~~~~~\biggl( \frac{a_n}{R_\mathrm{norm}} \biggr)^{4-3\gamma}</math>	<math>~=</math>	<math>~4\pi \biggl( \frac{\gamma-1}{\gamma} \biggr) \biggl( \frac{4\pi }{3} \cdot \mathfrak{f}_M \cdot \xi_1^3\biggr)^{\gamma-2} \, . </math>

Dependence on Size

Having completed carrying out the relevant volume integrals for a system of a particular size (radius), <math>~R,</math> we now establish how each term in the free-energy expression varies with system size. The size dependence will be expressed in terms of the normalized (and dimensionless) radius,

<math>~\chi \equiv \frac{R}{R_\mathrm{norm}} \, ,</math>

and, at the same time, we will normalize each energy term by dividing through by <math>~E_\mathrm{norm}.</math>

Mass Conservation: If the total mass of the system is held constant during a phase of contraction/expansion, the gravitational potential energy will scale as <math>~R^{-1},</math> specifically,

<math>~\frac{W_\mathrm{grav}}{E_\mathrm{norm}}</math>

<math>~=</math>

<math>~ - \frac{3}{5} \biggl( \frac{GM_\mathrm{tot}^2}{E_\mathrm{norm} R_\mathrm{norm}} \biggr) \frac{\mathfrak{f}_W}{\mathfrak{f}_M^2} \cdot \chi^{-1} = - \frac{3}{5} \frac{\mathfrak{f}_W}{\mathfrak{f}_M^2} \cdot \chi^{-1} \, .</math>

Angular Momentum Conservation: If, in addition, the system conserves its total angular momentum, <math>~J,</math> during a phase of contraction/expansion, we should write the expression for <math>~T_\mathrm{kin}</math> in terms of <math>~J.</math> For a system in uniform rotation, as is the case being considered here, we know that,

<math> T_\mathrm{rot} = \frac{1}{2}I \dot\varphi_c^2 ~~~~~ \mathrm{and} ~~~~~ J = I\dot\varphi_c \, , </math>

<math> \Rightarrow ~~~~ T_\mathrm{rot} = \frac{1}{2} \biggl( \frac{J^2}{I} \biggr) \, , </math>

where, from our above expression for <math>~T_\mathrm{kin}</math> we deduce that the scalar moment of inertia is

<math>~I</math>

<math>~=</math>

<math>~ \biggl( \frac{2}{5} M_\mathrm{tot} R^2 \biggr) \frac{\mathfrak{f}_T }{\mathfrak{f}_M} \, .</math>

[See also the formal definition of <math>~I</math> provided in our discussion of the scalar virial theorem; or see Eqs. (3) & (5) on p. 16 of EFE.] Hence, we can write,

<math>~\frac{T_\mathrm{kin}}{E_\mathrm{norm}} = \frac{T_\mathrm{rot}}{E_\mathrm{norm}}</math>

<math>~=</math>

<math>~ \frac{5}{4} \biggl( \frac{J^2}{M_\mathrm{tot} R_\mathrm{norm}^2 E_\mathrm{norm}} \biggr) \frac{\mathfrak{f}_M}{\mathfrak{f}_T} \cdot \chi^{-2} \, .</math>

Isothermal Contraction/Expansion: When a system expands or contracts isothermally, we will adopt the following as we develop an expression for the normalized <math>~\mathfrak{S}_I</math>: The sound speed, <math>~c_s,</math> is held constant; the normalized energy, <math>~E_\mathrm{norm}\rightarrow M_\mathrm{tot} c_s^2</math>; the previously unspecified normalization density, <math>~\rho_0</math>, will be replaced by <math>~\rho_\mathrm{norm}</math>; and mass conservation implies,

<math> \frac{\rho_c}{\rho_\mathrm{norm}} = \frac{\rho_c}{\bar\rho} \cdot \frac{\bar\rho}{\rho_\mathrm{norm}} = \mathfrak{f}_M^{-1} \cdot \chi^{-3} \, . </math>

Hence, to within an additive constant we can write,

<math>~\frac{\mathfrak{S}_I}{E_\mathrm{norm}}</math>

<math>~=</math>

<math>~ -3\ln\chi \, .</math>

Adiabatic Contraction/Expansion: If the system expands or contracts adiabatically, this means that <math>~K</math> — which specifies the material's specific entropy — is held constant. Therefore, the expression for <math>~\mathfrak{S}_A</math> must be written in terms of <math>~K</math>, rather than in terms of <math>~P_c</math>, in order to identify its proper variation with the system size. Appreciating that the relationship between <math>~K</math> and <math>~P_c</math> is governed by the (in this case, polytropic) equation of state,

<math> ~P_c = K \rho_c^{\gamma_g} \, , </math>

we can write,

<math>~\mathfrak{S}_A</math>	<math>~=</math>	<math>~ \frac{M_\mathrm{tot}}{({\gamma_g}-1)} \biggl[ K\rho_c^{\gamma_g-1}\biggr] \cdot \frac{\mathfrak{f}_A}{\mathfrak{f}_M} = \frac{M_\mathrm{tot} K}{({\gamma_g}-1)} \biggl[ \frac{3M_\mathrm{tot}}{4\pi R^3} \cdot \frac{1}{\mathfrak{f}_M} \biggr]^{\gamma_g-1} \cdot \frac{\mathfrak{f}_A}{\mathfrak{f}_M} </math>
	<math>~=</math>	<math>\frac{M_\mathrm{tot} K}{({\gamma_g}-1)} \biggl[ \frac{3M_\mathrm{tot}}{4\pi R_0^3} \biggr]^{\gamma_g-1} \biggl( \frac{R}{R_0} \biggr)^{-3(\gamma_g-1)} \cdot \frac{\mathfrak{f}_A}{\mathfrak{f}_M^{\gamma_g}} \, .</math>

Gathering it all Together

Gathering all of the terms together we find that, to within an additive constant, the expression for the free energy is,

<math> \mathfrak{G} = -3A\biggl( \frac{R}{R_0} \biggr)^{-1} -~ (1-\delta_{1\gamma_g})\frac{B}{(1-\gamma_g)}\biggl( \frac{R}{R_0} \biggr)^{3-3\gamma_g} -~ \delta_{1\gamma_g} 3B_I \ln \biggl( \frac{R}{R_0} \biggr) +~ C \biggl( \frac{R}{R_0} \biggr)^{-2} +~ D\biggl( \frac{R}{R_0} \biggr)^3 \, , </math>

where,

<math>~A</math>	<math>~\equiv</math>	<math>\frac{1}{5} \frac{GM_\mathrm{tot} ^2}{R_0} \cdot \frac{\mathfrak{f}_W}{\mathfrak{f}_M^2} \, ,</math>
<math>~B</math>	<math>~\equiv</math>	<math> K M_\mathrm{tot} \biggl( \frac{3M_\mathrm{tot} }{4\pi R_0^3} \biggr)^{\gamma_g - 1} \cdot \frac{\mathfrak{f}_A}{\mathfrak{f}_M^{\gamma_g}} = \bar{c_s}^2 M_\mathrm{tot} \cdot \frac{\mathfrak{f}_A}{\mathfrak{f}_M^{\gamma_g}} \, , </math>
<math>~B_I</math>	<math>~\equiv</math>	<math> c_s^2 M_\mathrm{tot} \, , </math>
<math>~C</math>	<math>~\equiv</math>	<math> \frac{5J^2}{4M_\mathrm{tot} R_0^2} \cdot \frac{\mathfrak{f}_M}{\mathfrak{f}_T} \, , </math>
<math>~D</math>	<math>~\equiv</math>	<math> \frac{4}{3} \pi R_0^3 P_e \, . </math>

Once the pressure exerted by the external medium (<math>~P_e</math>), and the configuration's mass (<math>~M_\mathrm{tot}</math>), angular momentum (<math>~J</math>), and specific entropy (via <math>~K</math>) — or, in the isothermal case, sound speed (<math>~c_s</math>) — have been specified, the values of all of the coefficients are known and the above algebraic expression for <math>~\mathfrak{G}</math> describes how the free energy of the configuration will vary with the configuration's size (<math>~R</math>) for a given choice of <math>~\gamma_g</math>.

Visual Representation

Figure 1: Free Energy Surface

This segment of the free energy "surface" shows how the free energy varies as the size of the configuration and the applied external pressure are varied, while all other relevant physical attributes are held fixed. The plotted function — derived from the above expression for <math>\mathfrak{G}</math>, with <math>\gamma_\mathrm{g} = 1</math> and <math>C=0</math> (see further discussion, below) — is, specifically, <math> \frac{\mathfrak{G}}{3Mc_s^2} = 3000\biggl[ - \frac{1}{\chi} - \ln\chi + \frac{\Pi}{3}\chi^3 + 0.9558 \biggr] \, . </math> As shown, the size of the configuration <math>(\chi)</math> increases to the right from <math>1.2</math> to <math>1.51</math>; the dimensionless external pressure <math>(\Pi)</math> increases into the screen from <math>0.103</math> to <math>0.104</math>; and the dimensionless free energy, <math>\mathfrak{G}/(3Mc_s^2)</math>, increases upward.

Energy Extrema

As is illustrated in Figure 1, the free energy surface generally will exhibit multiple local minima and local maxima, and may also possess one or more points of inflection. The locations along the energy surface where these special points arise identify equilibrium states, and the associated values of <math>(R/R_0)_\mathrm{eq}</math> give the radii of the equilibrium configurations.

For a given choice of the set of physical parameters <math>M</math>, <math>K</math>, <math>J</math>, <math>P_e</math>, and <math>\gamma_g</math>, extrema occur wherever,

<math> \frac{d\mathfrak{G}}{dR} = 0 \, . </math>

For the free energy function identified above,

<math> \frac{d\mathfrak{G}}{dR} = \frac{1}{R_0} \biggl[ 3A\chi^{-2} -~ (1-\delta_{1\gamma_g})~3 B\chi^{2 -3\gamma_g} -~ \delta_{1\gamma_g} 3B_I \chi^{-1} ~ -2C \chi^{-3} +~ 3D\chi^2 \biggr] \, . </math>

where,

<math>\chi \equiv \frac{R}{R_0} \, .</math>

So <math>\chi_\mathrm{eq} \equiv (R/R_0)_\mathrm{eq}</math> is obtained from the real root(s) of the equation,

<math> 2C \chi_\mathrm{eq}^{-2} + ~ (1-\delta_{1\gamma_g})~3 B\chi_\mathrm{eq}^{3 -3\gamma_g} +~ \delta_{1\gamma_g} 3B_I ~ -~3A\chi_\mathrm{eq}^{-1} -~ 3D\chi_\mathrm{eq}^3 = 0 \, . </math>

Examples

• Polytropic Spheres
  • Isolated, Nonrotating Configuration
  • Nonrotating Configuration Embedded in an External Medium

• Isothermal Spheres
  • Isolated, Nonrotating Configuration
  • Nonrotating Configuration Embedded in an External Medium

---

Material that appears after this point in our presentation is under development and therefore
may contain incorrect mathematical equations and/or physical misinterpretations.
|   Go Home   |

---

BiPolytrope

[Following a discussion that Tohline had with Kundan Kadam on 3 July 2013, we have decided to carry out a virial equilibrium and stability analysis of nonrotating bipolytropes.]

We will adopt the following approach:

• Properties of the core <math>\cdots</math>
  • Uniform density, <math>\rho_c</math>;
  • Polytropic constant, <math>K_c</math>, and polytropic index, <math>n_c</math>;
  • Surface of the core at <math>r_i</math>;
• Properties of the envelope <math>\cdots</math>
  • Uniform density, <math>\rho_e</math>;
  • Polytropic constant, <math>K_e</math>, and polytropic index, <math>n_e</math>;
  • Base of the core at <math>r_i</math> and surface at <math>R</math>.

Use the dimensionless radius,

<math>\xi \equiv \frac{r}{r_i}</math>.

Then, <math>\xi_i = 1</math> and <math>\xi_s \equiv R/r_i</math>.

Expressions for Mass

Inside the core, the expression for the mass interior to any radius, <math>0 \le \xi \le 1</math>, is,

<math>M_\xi = \frac{4\pi}{3} \rho_c r_i^3 \xi^3</math> .

The expression for the mass interior to any position within the envelope, <math>1 \le \xi \le \xi_s</math>, is,

<math>M_\xi = \frac{4\pi}{3} r_i^3 \biggl[\rho_c + \rho_e(\xi^3 - 1) \biggr]</math> .

Hence, in terms of a reference mass, <math>~M_0 \equiv 4\pi \rho_0 R_0^3/3</math>, the mass of the core, the mass of the envelope, and the total mass are, respectively,

<math>~M_\mathrm{core}</math>	<math>~=</math>	<math> \frac{4\pi}{3} \rho_c r_i^3 = M_0 \biggl[ \frac{\rho_c}{\rho_0} \biggl( \frac{r_i}{R_0}\biggr)^3 \biggr] ~~~~~\Rightarrow~~~~~ \frac{\rho_c}{\rho_0} = \frac{M_\mathrm{core}}{M_0} \biggl( \frac{r_i}{R_0}\biggr)^{-3} \, ; </math>
<math>~M_\mathrm{env}</math>	<math>~=</math>	<math> \frac{4\pi}{3} r_i^3 \biggl[\rho_e (\xi_s^3 - 1) \biggr] = M_0 (\xi_s^3 - 1) \biggl[ \frac{\rho_e}{\rho_0} \biggl( \frac{r_i}{R_0}\biggr)^3 \biggr] ~~~~~\Rightarrow~~~~~ \frac{\rho_e}{\rho_0} = \frac{M_\mathrm{env}}{M_0} \biggl( \frac{r_i}{R_0}\biggr)^{-3} (\xi_s^3 - 1)^{-1}\, ; </math>
<math>~M_\mathrm{tot}</math>	<math>~=</math>	<math> \frac{4\pi}{3} r_i^3 \biggl[\rho_c + \rho_e(\xi_s^3 - 1) \biggr] = M_0 \biggl( \frac{\rho_c}{\rho_0} \biggr) \biggl( \frac{r_i}{R_0}\biggr)^3 \biggl[ 1 + \frac{\rho_e}{\rho_c} (\xi_s^3 - 1) \biggr] \, . </math>

Following the work of Schönberg & Chandrasekhar (1942) — see our accompanying discussion — we will discuss bipolytropic equilibrium configurations in the context of a <math>~\nu - q</math> plane where,

<math>~\nu</math>	<math>~\equiv</math>	<math>~\frac{M_\mathrm{core}}{M_\mathrm{tot}} \, ,</math>
<math>~q</math>	<math>~\equiv</math>	<math>~\frac{r_i}{R} = \frac{1}{\xi_s} \, .</math>

With this in mind we can write,

<math>\frac{\rho_e}{\rho_c} = \frac{M_\mathrm{env}}{M_\mathrm{core}} (\xi_s^3 - 1)^{-1} = \frac{q^3 (1-\nu)}{\nu (1-q^3)} </math> ,

and,

<math>\nu \biggl(\frac{1-q^3}{q^3}\biggr) \biggl( \frac{\rho_e}{\rho_c} \biggr) = (1-\nu) ~~~~~\Rightarrow~~~~~ \nu = \biggl[ 1 + \biggl( \frac{\rho_e}{\rho_c} \biggr) \biggl(\frac{1-q^3}{q^3}\biggr) \biggr]^{-1} \, .</math>

Energy Expressions

The gravitational potential energy of the bipolytropic configuration is obtained by integrating over the following differential energy contribution,

<math>dW_\mathrm{grav} = - \biggl( \frac{GM_r}{r} \biggr) dm</math> .

Hence,

<math>~W_\mathrm{grav} = W_\mathrm{core} + W_\mathrm{env}</math>	<math> = - G \biggl\{ \int_0^{r_i} \biggl( \frac{M_r}{r} \biggr) 4\pi r^2 \rho_c dr + \int^R_{r_i} \biggl( \frac{M_r}{r} \biggr) 4\pi r^2 \rho_e dr \biggr\} </math>
	<math> = - G \biggl\{ \int_0^1 \biggl( \frac{4\pi }{3} \rho_c r_i^3 \xi^3 \biggr) 4\pi r_i^2 \rho_c \xi d\xi + \int_1^{\xi_s} \frac{4\pi}{3} \rho_c r_i^3 \biggl[ 1 + \frac{\rho_e}{\rho_c}(\xi^3 - 1) \biggr] 4\pi r_i^2 \rho_e \xi d\xi \biggr\} </math>
	<math> = - \frac{3GM^2_\mathrm{core}}{r_i} \biggl\{ \int_0^1 \xi^4 d\xi + \int_1^{\xi_s} \biggl[ 1 + \frac{\rho_e}{\rho_c}(\xi^3 - 1) \biggr] \biggl( \frac{\rho_e}{\rho_c} \biggr) \xi d\xi \biggr\} </math>
	<math> = - \frac{3GM^2_\mathrm{core}}{r_i} \biggl\{ \frac{1}{5} + \biggl( \frac{\rho_e}{\rho_c} \biggr) \int_1^{\xi_s} \xi d\xi + \biggl( \frac{\rho_e}{\rho_c} \biggr)^2 \int_1^{\xi_s} (\xi^3 - 1) \xi d\xi \biggr\} </math>
	<math> = - \frac{3GM^2_\mathrm{tot}}{R} \biggl( \frac{M_\mathrm{core}}{M_\mathrm{tot}} \biggr)^2 \xi_s \biggl\{ \frac{1}{5} + \frac{1}{2} \biggl( \frac{\rho_e}{\rho_c} \biggr) (\xi_s^2 - 1) + \biggl( \frac{\rho_e}{\rho_c} \biggr)^2 \biggl[ \frac{1}{5}(\xi_s^5 - 1) - \frac{1}{2}(\xi_s^2-1) \biggr] \biggr\} </math>

I like the form of this expression. The leading term, which scales as <math>~R^{-1}</math>, encapsulates the behavior of the gravitational potential energy for a given choice of the internal structure, namely, a given choice of <math>~\xi_s</math>, <math>~\nu</math>, and density ratio <math>~(\rho_e/\rho_c)</math>. Actually, only two internal structural parameters need to be specified — <math>~\xi_s</math> and <math>~f_c</math>; from these two, the expressions shown above allow the determination of both <math>~(\rho_e/\rho_c)</math> and <math>~\nu</math>. Keeping in mind our desire to discuss the properties of bipolytropes in the context of the <math>~\nu - q</math> plane introduced by Schönberg & Chandrasekhar (1942), we will rewrite this expression for the gravitational potential energy as,

<math>~W_\mathrm{grav}</math>

<math>~=</math>

<math>~- \frac{3}{5} \biggl( \frac{GM_\mathrm{tot}^2}{R} \biggr) \frac{\nu^2}{q} \cdot f\biggl(q, \frac{\rho_e}{\rho_c} \biggr) \, ,</math>

where,

<math>~f\biggl(q, \frac{\rho_e}{\rho_c} \biggr)</math>	<math>~\equiv</math>	<math> 1 + \frac{5}{2} \biggl( \frac{\rho_e}{\rho_c} \biggr) \biggl(\frac{1}{q^2} - 1 \biggr) + \biggl( \frac{\rho_e}{\rho_c} \biggr)^2 \biggl[ \biggl( \frac{1}{q^5} - 1 \biggr) - \frac{5}{2}\biggl(\frac{1}{q^2} - 1 \biggr) \biggr] </math>
	<math>~=</math>	<math> 1 + \frac{5}{2} \biggl( \frac{\rho_e}{\rho_c} \biggr) \frac{1}{q^5} \biggl[ (q^3- q^5 ) + \biggl( \frac{\rho_e}{\rho_c} \biggr) \biggl( \frac{2}{5} -q^3 + \frac{3}{5}q^5\biggr) \biggr] \, . </math>

© 2014 - 2021 by Joel E. Tohline |   H_Book Home   |   YouTube   | Appendices: | Equations | Variables | References | Ramblings | Images | myphys.lsu | ADS | 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