Editing SSCpt1/Virial (section)

=BiPolytrope=
[Following a discussion that Tohline had with Kundan Kadam on <font color="red">3 July 2013</font>, 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,
<div align="center">
<math>\xi \equiv \frac{r}{r_i}</math>.
</div>
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,
<div align="center">
<math>M_\xi = \frac{4\pi}{3} \rho_c r_i^3 \xi^3</math> .
</div>
The expression for the mass interior to any position within the envelope, <math>1 \le \xi \le \xi_s</math>, is,
<div align="center">
<math>M_\xi = \frac{4\pi}{3} r_i^3 \biggl[\rho_c  + \rho_e(\xi^3 - 1) \biggr]</math> .
</div>
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,

<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>M_\mathrm{core}</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<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>
  </td>
</tr>

<tr>
  <td align="right">
<math>M_\mathrm{env}</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<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>
  </td>
</tr>

<tr>
  <td align="right">
<math>M_\mathrm{tot}</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<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>
  </td>
</tr>
</table>
</div>

Following the work of [http://adsabs.harvard.edu/abs/1942ApJ....96..161S Sch&ouml;nberg &amp; Chandrasekhar (1942, ApJ, 96, 1615)] &#8212; see [[SSC/Structure/LimitingMasses#Sch.C3.B6nberg-Chandrasekhar_Mass|our accompanying discussion]]  &#8212; we will discuss bipolytropic equilibrium configurations in the context of a <math>\nu - q</math> plane where,
<table align="center" border="0" cellpadding="10">
<tr>
  <td align="right">
<math>\nu</math>
  </td>
  <td align="center">
<math>\equiv</math>
  </td>
  <td align="left">
<math>\frac{M_\mathrm{core}}{M_\mathrm{tot}} \, ,</math>
  </td>
</tr>

<tr>
  <td align="right">
<math>q</math>
  </td>
  <td align="center">
<math>\equiv</math>
  </td>
  <td align="left">
<math>\frac{r_i}{R} = \frac{1}{\xi_s} \, .</math>
  </td>
</tr>
</table>

With this in mind we can write,
<div align="center">
<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> ,
</div>
and,
<div align="center">
<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>
</div>

==Energy Expressions==

The gravitational potential energy of the bipolytropic configuration is obtained by integrating over the following differential energy contribution,
<div align="center">
<math>dW_\mathrm{grav} = - \biggl( \frac{GM_r}{r} \biggr) dm</math> .
</div>

Hence,
<table border="0" align="center">

<tr>
  <td align="right">
<math>W_\mathrm{grav} = W_\mathrm{core} + W_\mathrm{env}</math>
  </td>
  <td align="left">
<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>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="left">
<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>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="left">
<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>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="left">
<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>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="left">
<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>
  </td>
</tr>

</table>

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 &#8212; <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 [http://adsabs.harvard.edu/abs/1942ApJ....96..161S Sch&ouml;nberg &amp; Chandrasekhar (1942, ApJ, 96, 1615)], we will rewrite this expression for the gravitational potential energy as,
<div align="center">
<table border="0" cellpadding="5" align="center">
<tr>
  <td align="right">
<math>W_\mathrm{grav}</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<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>
  </td>
</tr>
</table>
</div>
where,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>f\biggl(q, \frac{\rho_e}{\rho_c} \biggr)</math>
  </td>
  <td align="center">
<math>\equiv</math>
  </td>
  <td align="left">
<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>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<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>
  </td>
</tr>
</table>
</div>