Editing SSC/Stability/BiPolytrope00CompareApproaches (section)

==Key Attributes of Equilibrium Configurations==
===Physical Properties===

[[File:CommentButton02.png|right|100px|This adopted parameter notation pays tribute to the notation that was introduced by Chandrasekhar and his collaborators in the early 1940s in papers associated with the discovery of the Sch&ouml;nberg-Chandrasekhar mass limit.]]Aside from specifying its radius, <math>~R</math>, and total mass, <math>~M_\mathrm{tot}</math>, there are three particularly interesting ''dimensionless'' parameters that characterize the internal structure of a bipolytrope having <math>~(n_c,n_e) = (0,0)</math>.  They are, the radial location of the core/envelope interface,
<div align="center">
<math>~q \equiv \frac{r_i}{R} \, ;</math>
</div>
the ratio of the density of the envelope material to the density of the core, <math>~0 \le \rho_e/\rho_c \le 1</math>; and the fraction of the total mass that is contained in the core,
<div align="center">
<math>~\nu \equiv \frac{M_\mathrm{core}}{M_\mathrm{tot}} \, .</math>
</div>
Identifying a unique bipolytropic configuration requires the specification of two of these three dimensionless parameters; the third parameter is, then, necessarily determined via what we will refer to as the,

<div align="center" id="PrimaryAlgebraicConstraint">
<font color="#770000">'''Primary Algebraic Constraint'''</font><br />
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~\frac{\rho_e}{\rho_c} </math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>~\frac{q^3(1-\nu)}{\nu(1-q^3)} \, .</math>
  </td>
</tr>
</table>
</div>
It is also relatively straightforward to appreciate that, in dimensional units, the value of the central density is,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~\rho_c</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\frac{3M_\mathrm{tot}}{4\pi G R^3} \cdot \frac{\nu}{q^3} \, .</math>
  </td>
</tr>
</table>
</div>

[[SSC/Structure/BiPolytropes/Analytic00#gdefinition|Our study of equilibrium configurations has shown]] that once, for example, the pair of parameters, <math>~q</math> and <math>~\rho_e/\rho_c</math>, has been specified, other properties of the associated equilibrium configuration can be succinctly expressed in terms of the function,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~g^2</math>
  </td>
  <td align="center">
<math>~\equiv</math>
  </td>
  <td align="left">
<math>
1  + \biggl(\frac{\rho_e}{\rho_c}\biggr)  \biggl[ 2 \biggl(1 - \frac{\rho_e}{\rho_c} \biggr) \biggl( 1-q \biggr) + 
\frac{\rho_e}{\rho_c} \biggl(\frac{1}{q^2} - 1\biggr) \biggr] \, .
</math>
  </td>
</tr>
</table>
</div>
For example, the central pressure is given by the expression,
<div align="center">
<table border="0">
<tr>
  <td align="right">
<math>~P_c</math>
  </td>
  <td align="center">
&nbsp; <math>~=</math>&nbsp; 
  </td>
  <td align="left">
<math>\biggl( \frac{3}{2^3\pi} \biggr) \frac{\nu^2 g^2}{q^4} \biggl[ \frac{GM_\mathrm{tot}^2}{R^4} \biggr] \, .</math>
  </td>
</tr>
</table>
</div>

===Sequences===
<table border="0" cellpadding="10" align="right"><tr><td align="center">
<table border="1" cellpadding="8" align="center">
<tr>
  <td align="center"><b>Figure 1:</b><br />Equilibrium Sequences of Constant <math>~\rho_e/\rho_c</math></td>
</tr>
<tr>
  <td align="center">
[[File:ConstDensitySequences.png|300px|Constant Density Sequences]]
  </td>
</tr>
</table>
</td></tr></table>
Across the two-dimensional, <math>~(q,\nu)</math> parameter space that is defined by the full range of physically viable values of <math>~q</math> and <math>~\nu</math>, namely,
<div align="center">
<math>~0 \le q \le 1 \, ,</math>&nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; <math>~0 \le \nu \le 1 \, ,</math>
</div>


an equilibrium model ''sequence'' can be defined by, for example, specifying that all models along the sequence have the same density jump at the interface.  Drawing on the above ''[[#PrimaryAlgebraicConstraint|primary algebraic constraint]]'', each choice of <math>~\rho_e/\rho_c</math> will generate a sequence governed by the function,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~\nu</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>~\biggl[\frac{(1-q^3)}{q^3} \biggl( \frac{\rho_e}{\rho_c} \biggr) + 1\biggr]^{-1} </math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>~\frac{q^3}{q^3 + (1-q^3)(\rho_e/\rho_c)} \, .</math>
  </td>
</tr>
</table>
</div>
Figure 1 displays several such equilibrium sequences across the <math>~(q,\nu)</math> plane &#8212; see also a [[SSC/Structure/BiPolytropes/Analytic00#Illustration|related figure associated with our free-energy determination of stability]].  The curves show how <math>~\nu</math> varies with <math>~q</math> along sequences for which the specified density ratio is <math>~\tfrac{1}{2}</math> (blue), <math>~\tfrac{1}{4}</math> (green), and <math>~\tfrac{1}{10}</math> (maroon).  We have employed a free-energy analysis (see summary, below) to examine whether a transition from stable to unstable configurations is encountered while traversing &#8212; that is, while ''evolving'' along &#8212; such sequences.

<table border="0" cellpadding="10" align="left"><tr><td align="center">
<table border="1" cellpadding="8" align="left">
<tr>
  <td align="center"><b>Figure 2:</b><br />Analytic Eigenvector Constraint</td>
</tr>
<tr>
  <td align="center">
[[File:EigenvectorSequence.png|300px|Analytic Eigenvector Sequence]]
  </td>
</tr>
</table>
</td></tr></table>
In a separate search for eigenvectors that simultaneously satisfy the linear adiabatic wave equation (LAWE) for the core and the LAWE for the envelope (see summary, below), we discovered that eigenvectors for some radial modes of oscillation can be specified ''fully analytically'' along a sequence of equilibrium models that is defined by what we will refer to as the,

<div align="center" id="AnalyticEigenvectorConstraint">
<font color="#770000">'''Analytic Eigenvector Constraint'''</font><br />
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~g^2</math>
  </td>
  <td align="center">
<math>~\equiv</math>
  </td>
  <td align="left">
<math>~1  + 2\biggl(\frac{\rho_e}{\rho_c}\biggr)  - 3\biggl(\frac{\rho_e}{\rho_c}\biggr)^2  
\, .
</math>
  </td>
</tr>
</table>
</div>
When combined with the above ''[[#PrimaryAlgebraicConstraint|primary algebraic constraint]]'', this is equivalent to demanding that,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~\nu</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\tfrac{1}{3}(1+2q^3) \, ,</math>
  </td>
</tr>
</table>
</div>
and, simultaneously,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~\frac{\rho_e}{\rho_c}</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\frac{2q^3}{1+2q^3} \, .</math>
  </td>
</tr>
</table>
</div>

The behavior of these two functions is displayed in Figure 2; the variation of <math>~\nu</math> with <math>~q</math> is traced by the dark blue squares while the variation of <math>~\rho_e/\rho_c</math> with <math>~q</math> is marked by the small, circular black dots.