View source for User:Tohline/SSC/FreeEnergy/Equilibrium Sequence Instabilities

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=Instabilities Associated with Equilibrium Sequence Turning Points=

The discussion presented here is supported by detailed reviews and new derivations presented in the following associated chapters: 
* A [[User:Tohline/SSC/FreeEnergy/PolytropesEmbedded#Free-Energy_Synopsis| free-energy synopsis]].  
* Material associated with an [[User:Tohline/SSC/FreeEnergy/PowerPoint#Supporting_Derivations_for_Free-Energy_PowerPoint_Presentation|overarching PowerPoint presentation]].
* [[User:Tohline/SSC/Structure/LimitingMasses#Mass_Upper_Limits|Limiting Masses]]
* An [[User:Tohline/SSC/Structure/BonnorEbert#Pressure-Bounded_Isothermal_Sphere|accompanying chapter on the Bonnor-Ebert sphere]].



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==Introduction==

Spherically symmetric, self-gravitating, equilibrium configurations can be constructed from gases exhibiting a wide variety of degrees of compressibility.  When examining how the internal structure of such configurations varies with compressibility, or when examining the relative stability of such structures, it can be instructive to construct models using a polytropic equation of state because the degree of compressibility can be adjusted by simply changing the value of the polytropic index, {{User:Tohline/Math/MP_PolytropicIndex}}, across the range, <math>~0 \le n \le \infty</math>.  (Alternatively, one can vary the effective adiabatic exponent of the gas, <math>~\gamma_g = 1 + 1/n</math>.)  In particular, <math>~n = 0 ~~ (\gamma_g = \infty)</math> represents a ''hard'' equation of state and describes an incompressible configuration, while <math>~n = \infty ~~(\gamma_g = 1)</math> represents an isothermal and extremely ''soft'' equation of state.

===Isolated Polytropes===
[[User:Tohline/SSC/Structure/Polytropes#Polytropic_Spheres|Isolated polytropic spheres]] exhibit three attributes that are especially key in the context of our present discussion:
<ol>
  <li>The equilibrium structure is dynamically stable if <math>~n < 3</math>.
  <li>The equilibrium structure has a finite radius if <math>~n < 5</math>.
  <li>The equilibrium structure can be described in terms of closed-form analytic expressions for [[User:Tohline/SSC/Structure/Polytropes#n_.3D_0_Polytrope|<math>~n = 0</math>]], 
[[User:Tohline/SSC/Structure/Polytropes#n_.3D_1_Polytrope|<math>~n = 1</math>]], and 
[[User:Tohline/SSC/Structure/Polytropes#n_.3D_5_Polytrope|<math>~n = 5</math>]].
</ol>

===Isothermal Spheres===
Isothermal spheres (polytropes with index <math>~n=\infty</math>) are discussed in a wide variety of astrophysical contexts because it is not uncommon for physical conditions to conspire to create an extended volume throughout which a configuration exhibits uniform temperature.  But [[User:Tohline/SSC/Structure/IsothermalSphere#Isothermal_Sphere|mathematical models of isothermal spheres]] are relatively cumbersome to analyze because they extend to infinity, they are dynamically unstable, and they are not describable in terms of analytic functions.  In such astrophysical contexts, we have sometimes found it advantageous to employ an <math>~n=5</math> polytrope instead of an isothermal sphere.  An [[User:Tohline/SSC/Structure/Polytropes#n_.3D_5_Polytrope|isolated <math>~n=5</math> polytrope]] can serve as an effective surrogate for an isothermal sphere because it is infinite in extent and is dynamically unstable, but it is less cumbersome to analyze because its structure can be described by closed-form analytic expressions.

===Bounded Isothermal Sphere & Bonnor-Ebert Mass===
In the mid-1950s, [http://adsabs.harvard.edu/abs/1955ZA.....37..217E Ebert] (1955) and [http://adsabs.harvard.edu/abs/1956MNRAS.116..351B Bonnor] (1956) independently realized that an isothermal gas cloud can be stabilized by embedding it in a hot, tenuous external medium.  The relevant mathematical model is constructed by chopping off the isothermal sphere at some finite radius &#8212; call it, <math>~\xi_e</math> &#8212; and imposing an externally applied pressure, <math>~P_e</math>, that is equal to the pressure of the isothermal gas at the specified edge of the truncated sphere.  But for a given mass and temperature, there is a value of <math>~P_e</math> below which the truncated isothermal sphere is dynamically unstable, like its isolated and untruncated counterpart.  Viewed another way, given the value of <math>~P_e</math> and the isothermal sound speed, <math>~c_s</math>, a bounded isothermal sphere will be dynamically stable only if its mass is below a critical value, 
<div align="center">
<table border="0" cellpadding="5" width="95%">
<tr>
  <td align="center">
'''<font color="red">Bonnor-Ebert Mass</font>'''
<br>&nbsp;<br>
<math>M_\mathrm{max} = \alpha \biggl( \frac{c_s^8}{G^3 P_e} \biggr)^{1/2}</math>
  </td>
  <td align="center">

<!-- Begin inset table-->
<table border="1" cellpadding="5">
<tr>
  <th align="center" colspan="3">
Coefficient <math>\alpha</math> for Pressure-Bounded Configurations
  </th>
</tr>
<tr>
  <th align="center">
<font size="+1">
<math>~\alpha</math>
</font>
  </th>
  <th align="center">
Context
  </th>
  <th align="center">
Source
  </th>
</tr>

<tr>
  <td align="center">
<math>~1.18</math>
  </td>
  <td align="center">
<font size="-2">
Bounded Isothermal Sphere
</font>
<br>
<font size="-2">
(numerically derived)
</font>
  </td>
  <td align="center">
<font size="-2">Discovery Paper</font>
<br>
[http://adsabs.harvard.edu/abs/1956MNRAS.116..351B Bonnor] (1956)
<br>
<font size="-2">(see also [[User:Tohline/SSC/Structure/BonnorEbert#Maximum_Mass|here]])</font>
  </td>
</tr>

<tr>
  <td align="center">
<math>\biggl( \frac{3^4\cdot 5^3}{2^{10}\pi} \biggr)^{1/2}</math>
  </td>
  <td align="center">
<font size="-2">
Isothermal Virial Analysis
</font>
<br>
<font size="-2">
(exact)
</font>
  </td>
  <td align="center">
[[User:Tohline/SphericallySymmetricConfigurations/Virial#Bounded_Isothermal|Here]]
  </td>
</tr>

<tr>
  <td align="center">
<math>\biggl( \frac{1}{2} \biggr)^{3/10}\biggl( \frac{3^{7}}{2^{8}\pi} \biggr)^{1/2}</math>
  </td>
  <td align="center">
<font size="-2">
Bounded <math>~n=5</math> Polytrope
</font>
<br>
<font size="-2">
(exact)
</font>
  </td>
  <td align="center">
[[User:Tohline/SSC/Structure/PolytropesEmbedded#n_.3D_5_Polytrope|Here]]
  </td>
</tr>

<tr>
  <td align="center">
<math>\biggl( \frac{3^{19}}{2^{12}\cdot 5^7\pi} \biggr)^{1/2}</math>
  </td>
  <td align="center">
<font size="-2">
<math>~n=5</math> Virial Analysis
</font>
<br>
<font size="-2">
(exact)
</font>
  </td>
  <td align="center">
[[User:Tohline/SphericallySymmetricConfigurations/Virial#Bounded_Adiabatic|Here]]
  </td>
</tr>
</table>
<!-- End inset table-->

  </td>
</tr>
</table>
</div>
where <math>~\alpha</math> is a dimensionless coefficient of order unity.  This limiting mass is often referred to as the Bonnor-Ebert mass.  It appears most frequently in the astrophysics literature in discussions of star formation because that is the arena in which both Bonnor and Ebert were conducting research when they made their discoveries.  

As is [[User:Tohline/SSC/Structure/BonnorEbert#Maximum_Mass|reviewed in a related discussion]] and as is documented in the table accompanying the expression for <math>~M_\mathrm{max}</math>, above, [http://adsabs.harvard.edu/abs/1956MNRAS.116..351B Bonnor] (1956) used [[User:Tohline/SSC/Structure/IsothermalSphere#Emden.27s_Numerical_Solution|Emden's (1907) tabulated properties]] of an isothermal sphere to determine that the dimensionless radius of this limiting configuration is <math>\xi_e \approx 6.5</math> and that the leading coefficient, <math>\alpha \approx 1.18</math>.  It is worth noting that a [[User:Tohline/SphericallySymmetricConfigurations/Virial#BonnorEbertMass|global virial analysis of the stability of bounded isothermal spheres]] produces the same expression for <math>~M_\mathrm{max}</math> with a leading coefficient that has an exact, analytic prescription, namely, <math>\alpha = (3^4 \cdot 5^3/2^{10}\pi)^{1/2} \approx 1.77408</math>.  While it can be advantageous to reference this analytic prescription of <math>\alpha</math>, the virial analysis must be considered more approximate than Bonnor's analysis because it does not require the construction of models that are in detailed force balance.  

Our [[User:Tohline/SSC/Structure/PolytropesEmbedded#Extension_to_Bounded_Sphere_2|detailed force-balance analysis of truncated and pressure-bounded, <math>~n=5</math> polytropes]] identifies a physically analogous limiting mass.  If the average isothermal sound speed, <math>~\bar{c_s}</math>, [[User:Tohline/SphericallySymmetricConfigurations/Virial#average|as defined elsewhere]], is used in place of <math>c_s</math>, the mathematical expression for <math>~M_\mathrm{max}</math> has exactly the same form as in the isothermal case.  But for the <math>~n=5</math> polytrope we know that the limiting configuration has a dimensionless radius given precisely by <math>~\xi_e = 3</math>; and, as a result, the leading coefficient in the definition of <math>~M_\mathrm{max}</math> is prescribable analytically, namely, <math>~\alpha = 2^{-3/10} \cdot (3^7/2^8 \pi)^{1/2} \approx 1.33943</math>.  As is documented in the table accompanying the relation for <math>~M_\mathrm{max}</math>, above, in the case of a truncated <math>~n=5</math> polytrope, [[User:Tohline/SphericallySymmetricConfigurations/Virial#Maximum_Mass|the simpler and more approximate virial analysis gives]], <math>~\alpha = (3^{19}/2^{12}\cdot 5^7\pi)^{1/2} \approx 1.07523</math>.


==Bonnor-Ebert Sphere==

This information extracted from an [[User:Tohline/SSC/Structure/BonnorEbert#Pressure-Bounded_Isothermal_Sphere|accompanying chapter on the Bonnor-Ebert sphere]].


<div align="center">
<table border="1">
<tr>
  <td align="center" colspan="2">
Figures from (left) [http://adsabs.harvard.edu/abs/1955ZA.....37..217E Ebert (1955)] and (right) [http://adsabs.harvard.edu/abs/1956MNRAS.116..351B Bonnor (1956)] 
  </td>
</tr>
<tr>
  <td align="center">
[[File:EbertFig2Title.jpg|250px|center|Ebert (1955) Figure 2]]
  </td>
  <td align="center">
[[File:Bonnor1956Fig1reproduced.jpg|400px|center|Bonnor (1956, MNRAS, 116, 351)]]
<!-- [[Image:AAAwaiting01.png|400px|center|Bonnor (1956, MNRAS, 116, 351)]] -->
  </td>
</tr>
</table>
</div>


==Sch&ouml;nberg-Chandrasekhar Mass==


<div align="center">
<table border="1" cellpadding="5" width="80%">

<tr>
  <td align="center">
Edited excerpt from [http://adsabs.harvard.edu/abs/1942ApJ....96..161S Sch&ouml;nberg &amp; Chandrasekhar (1942)]
  </td>
  <td align="center">
Figure from [http://adsabs.harvard.edu/abs/1941ApJ....94..525H Henrich &amp; Chandraskhar (1941)]
  </td>
</tr>
<tr>
  <td align="center" rowspan="1">
[[Image:SC42_Fig1.jpg|400px|center]]
<!-- [[Image:AAAwaiting01.png|400px|center]] -->
  </td>
  <td align="center" rowspan="1">
[[Image:HenrichChandra41b.jpg|200px|center]]
<!-- [[Image:AAAwaiting01.png|200px|center]] -->
  </td>
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</table>
</div>


In the early 1940s, Chandrasekhar and his colleagues (see [http://adsabs.harvard.edu/abs/1941ApJ....94..525H Henrich &amp; Chandraskhar (1941)] and [http://adsabs.harvard.edu/abs/1942ApJ....96..161S Sch&ouml;nberg &amp; Chandrasekhar (1942)]) discovered that a star with an isothermal core will become unstable if the fractional mass of the core is above some limiting value.  They discovered this by constructing models that are now commonly referred to as ''composite polytropes'' or ''bipolytropes'', that is, models in which the star's core is described by a polytropic equation of state having one index &#8212; say, <math>~n_c</math> &#8212; and the star's envelope is described by a polytropic equation of state of a different index &#8212; say, <math>~n_e</math>.  In [[User:Tohline/SSC/Structure/BiPolytropes#BiPolytropes|an accompanying discussion]] we explain in detail how the two structural components with different polytropic indexes are pieced together mathematically to build equilibrium bipolytropes. For a given choice of the two indexes, <math>~n_c</math> and <math>~n_e</math>, a sequence of models can be generated by varying the radial location at which the interface between the core and envelope occurs.  As the interface location is varied, the relative amount of mass enclosed inside the core, <math>~\nu \equiv M_\mathrm{core}/M_\mathrm{tot}</math>, quite naturally varies as well. 

[http://adsabs.harvard.edu/abs/1941ApJ....94..525H Henrich &amp; Chandraskhar (1941)] built structures of uniform composition having an isothermal core (<math>~n_c = \infty</math>) and an <math>~n_e = 3/2</math> polytropic envelope and found that equilibrium models exist only for values of <math>~\nu \le \nu_\mathrm{max} \approx 0.35</math>.  [http://adsabs.harvard.edu/abs/1942ApJ....96..161S Sch&ouml;nberg &amp; Chandrasekhar (1942)] extended this analysis to include structures in which the mean molecular weight of the gas changes discontinuously across the interface.  Specifically, they used the same values of <math>~n_c</math> and <math>~n_e</math> as Henrich &amp; Chandrasekhar, but they constructed models in which the ratio of the molecular weight in the core to the molecular weight in the envelope is <math>~\mu_c/\mu_e = 2</math>. This was done to more realistically represent stars as they evolve off the main sequence; they have inert, isothermal helium cores and envelopes that are rich in hydrogen.  Note that introducing a discontinuous drop in the mean molecular weight at the core-envelope interface also introduces a discontinuous drop in the gas density across the interface.  As the following excerpt from p. 168 of their article summarizes, in these models, [http://adsabs.harvard.edu/abs/1942ApJ....96..161S Sch&ouml;nberg &amp; Chandrasekhar (1942)] found that <math>~\nu_\mathrm{max} \approx 0.101</math>.  This is commonly referred to as the Sch&ouml;nberg-Chandrasekhar mass limit, although it was Henrich &amp; Chandrasekhar who were the first to identify the instability.
<div align="center">
<table border="1" cellpadding="5" width="60%">
<tr>
  <td align="center" colspan="2">
<b>
Text excerpt from [http://adsabs.harvard.edu/abs/1942ApJ....96..161S Sch&ouml;nberg &amp; Chandrasekhar (1942)]
</b>
  </td>
</tr>

<tr>
  <td align="center" colspan="2">
[[Image:SC42excerpt.jpg|600px|center]]
<!-- [[Image:AAAwaiting01.png|600px|center]] -->
  </td>
</tr>

</table>
</div>
In an effort to develop a more complete appreciation of the onset of the instability associated with the Sch&ouml;nberg-Chandrasekhar mass limit, [http://adsabs.harvard.edu/abs/1988Ap%26SS.147..219B Beech (1988)] matched an analytically prescribable, <math>~n_e = 1</math> polytropic envelope to an isothermal core and, like Sch&ouml;nberg &amp; Chandrasekhar, allowed for a discontinuous change in the molecular weight at the interface.  [For an even more comprehensive generalization and discussion, see [http://adsabs.harvard.edu/abs/2012MNRAS.421.2713B Ball, Tout, &amp; &#x017B;ytkow] (2012, MNRAS, 421, 2713)].  Beech's results were not significantly different from those reported by [http://adsabs.harvard.edu/abs/1942ApJ....96..161S Sch&ouml;nberg &amp; Chandrasekhar (1942)]; in particular, the value of <math>~\nu_\mathrm{max}</math> was still only definable numerically because an isothermal core cannot be described in terms of analytic functions. 

[[User:Tohline/SSC/Structure/BiPolytropes/Analytic5_1#BiPolytrope_with_nc_.3D_5_and_ne_.3D_1|In an accompanying derivation]] [see, also, [http://adsabs.harvard.edu/abs/1998MNRAS.298..831E Eggleton, Faulkner, and Cannon] (1998, MNRAS, 298, 831)] we have gone one step farther, matching an analytically prescribable, <math>~n_e = 1</math> polytropic envelope to an analytically prescribable, <math>~n_c = 5</math> polytropic core.  For this bipolytrope, we show that there is a limiting mass-fraction, <math>~\nu_\mathrm{max}</math>, for any choice of the molecular weight ratio <math>~\mu_c/\mu_e > 3</math> and that the interface location, <math>~\xi_i</math>, associated with this critical configuration is given by the positive, real root of the following relation:

<div align="center">
<math>
\biggl(\frac{\pi}{2} + \tan^{-1} \Lambda_i\biggr) (1+\ell_i^2) [ 3 + (1-m_3)^2(2-\ell_i^2)\ell_i^2] -
m_3 \ell_i [(1-m_3)\ell_i^4 - (m_3^2 - m_3 +2)\ell_i^2 - 3] = 0 \, ,
</math> 
</div>
where,
<div align="center">
<math>
\ell_i \equiv \frac{\xi_i}{\sqrt{3}}  \, ;
</math>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 
<math>
m_3 \equiv 3 \biggl( \frac{\mu_c}{\mu_e} \biggr)^{-1} \, ;
</math>
&nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; 
<math>
\Lambda_i \equiv \frac{1}{m_3\ell_i} [ 1 + (1-m_3)\ell_i^2]  \, .
</math>
</div>

==Close Binary Stars==

* [http://adsabs.harvard.edu/abs/1993ApJ...406L..63L Lai, Rasio, &amp; Shapiro (1993a, ApJL, 406, L63)]
* [http://adsabs.harvard.edu/abs/1993ApJS...88..205L Lai, Rasio, &amp; Shapiro (1993b, ApJS, 88, 205)]
* [http://adsabs.harvard.edu/abs/1969ApJ…157.1395O Ostriker &amp; Gunn (1969, ApJ, 157, 1395)]

<!--
====Coincidence Between Points of Secular and Dynamical Instability====
From the [[User:Tohline/SSC/Structure/BiPolytropes/Analytic5_1#Model_Sequences|accompanying graphical display of equilibrium sequences]], it seems that a <math>~\nu_\mathrm{max}</math> turning point will only exist in five-one bipolytropes for <math>~\mu_e/\mu_c</math> less than some value &#8212; call it, <math>~(\mu_e/\mu_c)_\mathrm{begin}</math> &#8212; which is less than but approximately equal to <math>~\tfrac{1}{3}</math>.  As we move along any sequence for which <math>~\mu_e/\mu_c < (\mu_e/\mu_c)_\mathrm{begin}</math>, in the direction of increasing <math>~\ell_i</math>, it is fair to ask whether the system becomes dynamically unstable (at <math>~[x_\mathrm{eq}]_\mathrm{crit}</math>) before or after it encounters the point of secular instability marked by <math>~\nu_\mathrm{max}</math>.
-->

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