ParabolicDensity/Axisymmetric/Structure

Parabolic Density Distribution

Part I:   Gravitational Potential

Part II:   Spherical Structures

Part III:   Axisymmetric Equilibrium Structures

Part IV:   Triaxial Equilibrium Structures (Exploration)

Axisymmetric (Oblate) Equilibrium Structures

Setup

Here we specifically discuss the case of configurations that exhibit concentric ellipsoidal iso-density surfaces of the form,

$\rho$

=

${\rho }_c[1-(\frac{x^2+y^2}{a_{\ell }^2}+\frac{z^2}{a_s^2}\left)\right],$

that is, axisymmetric ($a_m=a_{\ell }$, i.e., oblate) configurations with parabolic density distributions. Much of our presentation, here, is drawn from our separate, detailed description of what we will refer to as Ferrers potential.

This can be rewritten in terms of T1 Coordinates. In particular, defining, $q\equiv a_{\ell }/a_s$ and, ${\xi }_1$ $\equiv$ $[z^2+(\frac{\varpi }{q}{)}^2{]}^{1/2}=a_s\left[\right(\frac{z}{a_s}{)}^2+(\frac{\varpi }{a_{\ell }}{)}^2{]}^{1/2}$ $\Rightarrow \frac{\rho }{{\rho }_c}$ $=$ $[1-(\frac{{\xi }_1}{a_s}{)}^2].$ Because we expect contours of constant enthalpy $(H)$ to coincide with contours of constant density in equilibrium configurations, we should expect to find that, $\frac{H}{H_c}$ $=$ $h({\xi }_1).$ If the "radial" enthalpy profile resembles our derived spherical enthalpy profile, we should expect to find that, $h({\xi }_1)$ $\sim$ $h_0[1-h_2{\xi }_1^2-h_4{\xi }_1^4]$ $\Rightarrow 1-\frac{h({\xi }_1)}{h_0}$ $\sim$ $h_2{\xi }_1^2+h_4{\xi }_1^4$   $=$ $h_2{a}_s^2\left[\right(\frac{z}{a_s}{)}^2+\left(\frac{\varpi }{a_{\ell }}{)}^2\right]+h_4\left\{a_s^2\right[(\frac{z}{a_s}{)}^2+(\frac{\varpi }{a_{\ell }}{)}^2]{\}}^2$   $=$ $h_2{a}_s^2\left[\right(\frac{z}{a_s}{)}^2+\left(\frac{\varpi }{a_{\ell }}{)}^2\right]+h_4{a}_s^4\left[\right(\frac{z}{a_s}{)}^4+2\left(\frac{z}{a_s}{)}^2\right(\frac{\varpi }{a_{\ell }}{)}^2+\left(\frac{\varpi }{a_{\ell }}{)}^4\right]$

Gravitational Potential

As we have detailed in an accompanying discussion, for an oblate-spheroidal configuration — that is, when $a_s<a_m=a_{\ell }$ — the gravitational potential may be obtained from the expression,

$\frac{{\mathrm{\Phi }}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}(\mathbf{𝐱})}{(-\pi G{\rho }_c)}$

$=$

$\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}{a}_1^2-(A_1{x}^2+A_2{y}^2+A_3{z}^2)+(A_{12}{x}^2{y}^2+A_{13}{x}^2{z}^2+A_{23}{y}^2{z}^2)+\frac{1}{6}(3A_{11}{x}^4+3A_{22}{y}^4+3A_{33}{z}^4),$

where, in the present context, we can rewrite this expression as,

$\frac{{\mathrm{\Phi }}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}(\mathbf{𝐱})}{(-\pi G{\rho }_c)}$	$=$	$\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}{a}_{\ell }^2-[A_{\ell }(x^2+y^2)+A_s{z}^2]+[A_{\ell \ell }{x}^2{y}^2+A_{\ell s}{x}^2{z}^2+A_{\ell s}{y}^2{z}^2]+\frac{1}{6}[3A_{\ell \ell }{x}^4+3A_{\ell \ell }{y}^4+3A_{ss}{z}^4]$
	$=$	$\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}{a}_{\ell }^2-[A_{\ell }{\varpi }^2+A_s{z}^2]+[A_{\ell \ell }{x}^2{y}^2+A_{\ell s}{\varpi }^2{z}^2]+\frac{1}{2}[A_{\ell \ell }(x^4+y^4)+A_{ss}{z}^4]$
	$=$	$\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}{a}_{\ell }^2-[A_{\ell }{\varpi }^2+A_s{z}^2]+\frac{A_{\ell \ell }}{2}[(x^2+y^2{)}^2]+\frac{1}{2}[A_{ss}{z}^4]+[A_{\ell s}{\varpi }^2{z}^2]$
	$=$	$\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}{a}_{\ell }^2-[A_{\ell }{\varpi }^2+A_s{z}^2]+\frac{A_{\ell \ell }}{2}\left[{\varpi }^4\right]+\frac{1}{2}\left[A_{ss}{z}^4\right]+\left[A_{\ell s}{\varpi }^2{z}^2\right]$
$\Rightarrow \frac{{\mathrm{\Phi }}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}(\mathbf{𝐱})}{(-\pi G{\rho }_c{a}_{\ell }^2)}$	$=$	$\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}-\left[A_{\ell }\right(\frac{{\varpi }^2}{a_{\ell }^2})+A_s(\frac{z^2}{a_{\ell }^2}\left)\right]+\frac{1}{2}\left[A_{\ell \ell }{a}_{\ell }^2\right(\frac{{\varpi }^4}{a_{\ell }^4})+A_{ss}{a}_{\ell }^2(\frac{z^4}{a_{\ell }^4})+2A_{\ell s}{a}_{\ell }^2(\frac{{\varpi }^2{z}^2}{a_{\ell }^4}\left)\right].$

Index Symbol Expressions

The expression for the zeroth-order normalization term $(I_{BT})$, and the relevant pair of 1^st-order index symbol expressions are:

$I_{\mathrm{B}\mathrm{T}}$	$=$	$2A_{\ell }+A_s(1-e^2)=2(1-e^2{)}^{1/2}[\frac{{\sin }^{-1}e}{e}];$
$A_{\ell }$	$=$	$\frac{1}{e^2}[\frac{{\sin }^{-1}e}{e}-(1-e^2{)}^{1/2}](1-e^2{)}^{1/2};$
$A_s$	$=$	$\frac{2}{e^2}[(1-e^2{)}^{-1/2}-\frac{{\sin }^{-1}e}{e}](1-e^2{)}^{1/2},$

[EFE], Chapter 3, Eq. (36)
[T78], §4.5, Eqs. (48) & (49)

where the eccentricity,

$e\equiv [1-(\frac{a_s}{a_{\ell }}{)}^2{]}^{1/2}.$

The relevant 2^nd-order index symbol expressions are:

$a_{\ell }^2{A}_{\ell \ell }$	$=$	$\frac{1}{4e^4}\{-(3+2e^2)(1-e^2)+3(1-e^2{)}^{1/2}\left[\frac{{\sin }^{-1}e}{e}\right]\};$
$\frac{3}{2}{a}_{\ell }^2{A}_{ss}$	$=$	$\frac{(4e^2-3)}{e^4(1-e^2)}+\frac{3(1-e^2{)}^{1/2}}{e^4}\left[\frac{{\sin }^{-1}e}{e}\right];$
$a_{\ell }^2{A}_{\ell s}$	$=$	$\frac{1}{e^4}\{(3-e^2)-3(1-e^2{)}^{1/2}\left[\frac{{\sin }^{-1}e}{e}\right]\}.$

We can crosscheck this last expression by drawing on a shortcut expression,

$A_{\ell s}$	$=$	$-\frac{A_{\ell }-A_s}{(a_{\ell }^2-a_s^2)}$
$\Rightarrow a_{\ell }^2{A}_{\ell s}$	$=$	$\frac{1}{e^2}\{A_s-A_{\ell }\}$
	$=$	$\frac{1}{e^2}\left\{\frac{2}{e^2}\right[(1-e^2{)}^{-1/2}-\frac{{\sin }^{-1}e}{e}](1-e^2{)}^{1/2}-\frac{1}{e^2}[\frac{{\sin }^{-1}e}{e}-(1-e^2{)}^{1/2}](1-e^2{)}^{1/2}\}$
	$=$	$\frac{1}{e^4}\left\{\right[2-2(1-e^2{)}^{1/2}\frac{{\sin }^{-1}e}{e}]-[(1-e^2{)}^{1/2}\frac{{\sin }^{-1}e}{e}-(1-e^2)\left]\right\}$
	$=$	$\frac{1}{e^4}\{(3-e^2)-3(1-e^2{)}^{1/2}\frac{{\sin }^{-1}e}{e}\}.$

Meridional Plane Equi-Potential Contours

Here, we follow closely our separate discussion of equipotential surfaces for Maclaurin Spheroids, assuming no rotation.

Configuration Surface

In the meridional $(\varpi ,z)$ plane, the surface of this oblate-spheroidal configuration — identified by the thick, solid-black curve below, in Figure 1 — is defined by the expression,

$\frac{\rho }{{\rho }_c}$	$=$	$1-[\frac{{\varpi }^2}{a_{\ell }^2}+\frac{z^2}{a_s^2}]=0$
$\Rightarrow \frac{{\varpi }^2}{a_{\ell }^2}+\frac{z^2}{a_s^2}$	$=$	$1$
$\Rightarrow z^2$	$=$	$a_s^2[1-\frac{{\varpi }^2}{a_{\ell }^2}]=a_{\ell }^2(1-e^2)[1-\frac{{\varpi }^2}{a_{\ell }^2}]$
$\Rightarrow \frac{z}{a_{\ell }}$	$=$	$\pm (1-e^2{)}^{1/2}[1-\frac{{\varpi }^2}{a_{\ell }^2}{]}^{1/2},$	for $0\le \frac{|\varpi |}{a_{\ell }}\le 1.$

Expression for Gravitational Potential

Throughout the interior of this configuration, each associated ${\mathrm{\Phi }}_{\mathrm{e}\mathrm{f}\mathrm{f}}$ = constant, equipotential surface is defined by the expression,

${\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}\equiv \frac{{\mathrm{\Phi }}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}(\mathbf{𝐱})}{(\pi G{\rho }_c{a}_{\ell }^2)}+\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}$

$=$

$\left[A_{\ell }\right(\frac{{\varpi }^2}{a_{\ell }^2})+A_s(\frac{z^2}{a_{\ell }^2}\left)\right]-\frac{1}{2}\left[A_{\ell \ell }{a}_{\ell }^2\right(\frac{{\varpi }^4}{a_{\ell }^4})+A_{ss}{a}_{\ell }^2(\frac{z^4}{a_{\ell }^4})+2A_{\ell s}{a}_{\ell }^2(\frac{{\varpi }^2{z}^2}{a_{\ell }^4}\left)\right].$

Letting,

$\zeta \equiv \frac{z^2}{a_{\ell }^2}$,

we can rewrite this expression for ${\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}$ as,

${\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}$	$=$	$A_{\ell }\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)+A_s\zeta -\frac{1}{2}{A}_{\ell \ell }{a}_{\ell }^2\left(\frac{{\varpi }^4}{a_{\ell }^4}\right)-\frac{1}{2}{A}_{ss}{a}_{\ell }^2{\zeta }^2-A_{\ell s}{a}_{\ell }^2\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)\zeta$
	$=$	$-\frac{1}{2}{A}_{ss}{a}_{\ell }^2{\zeta }^2+[A_s-A_{\ell s}{a}_{\ell }^2(\frac{{\varpi }^2}{a_{\ell }^2}\left)\right]\zeta +A_{\ell }\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)-\frac{1}{2}{A}_{\ell \ell }{a}_{\ell }^2\left(\frac{{\varpi }^4}{a_{\ell }^4}\right).$

Potential at the Pole

At the pole, $(\varpi ,z)=(0,a_s)$. Hence,

${\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}{|}_{\mathrm{m}\mathrm{i}\mathrm{d}}$	$=$	$-\frac{1}{2}{A}_{ss}{a}_{\ell }^2(\frac{a_s^2}{a_{\ell }^2}{)}^2+[A_s-A_{\ell s}{a}_{\ell }^2{\xcancel{\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)}}^0\left]\right(\frac{a_s^2}{a_{\ell }^2})+A_{\ell }{\xcancel{\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)}}^0-\frac{1}{2}{A}_{\ell \ell }{a}_{\ell }^2{\xcancel{\left(\frac{{\varpi }^4}{a_{\ell }^4}\right)}}^0$
	$=$	$A_s\left(\frac{a_s^2}{a_{\ell }^2}\right)-\frac{1}{2}{A}_{ss}{a}_{\ell }^2(\frac{a_s^2}{a_{\ell }^2}{)}^2.$

General Determination of Vertical Coordinate (ζ)

Given values of the three parameters, $e$, $\varpi$, and ${\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}$, this last expression can be viewed as a quadratic equation for $\zeta$. Specifically, $0$ $=$ $\alpha {\zeta }^2+\beta \zeta +\gamma ,$ where, $\alpha$ $\equiv$ $\frac{1}{2}{A}_{ss}{a}_{\ell }^2$   $=$ $\frac{1}{3}\{\frac{(4e^2-3)}{e^4(1-e^2)}+\frac{3(1-e^2{)}^{1/2}}{e^4}[\frac{{\sin }^{-1}e}{e}\left]\right\},$ $\beta$ $\equiv$ $A_{\ell s}{a}_{\ell }^2\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)-A_s$   $=$ $\frac{1}{e^4}\{(3-e^2)-3(1-e^2{)}^{1/2}\frac{{\sin }^{-1}e}{e}\left\}\right(\frac{{\varpi }^2}{a_{\ell }^2})-\frac{2}{e^2}[(1-e^2{)}^{-1/2}-\frac{{\sin }^{-1}e}{e}](1-e^2{)}^{1/2},$ $\gamma$ $\equiv$ ${\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}+\frac{1}{2}{A}_{\ell \ell }{a}_{\ell }^2\left(\frac{{\varpi }^4}{a_{\ell }^4}\right)-A_{\ell }\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)$   $=$ ${\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}+\frac{1}{8e^4}\{-(3+2e^2)(1-e^2)+3(1-e^2{)}^{1/2}\left[\frac{{\sin }^{-1}e}{e}\right]\left\}\right(\frac{{\varpi }^4}{a_{\ell }^4})-\frac{1}{e^2}[\frac{{\sin }^{-1}e}{e}-(1-e^2{)}^{1/2}](1-e^2{)}^{1/2}(\frac{{\varpi }^2}{a_{\ell }^2}).$ The solution of this quadratic equation gives, $\zeta$ $=$ $\frac{1}{2\alpha }\{-\beta \pm [{\beta }^2-4\alpha \gamma {]}^{1/2}\}.$ Should we adopt the superior (positive) sign, or is it more physically reasonable to adopt the inferior (negative) sign? As it turns out, $\beta$ is intrinsically negative, so the quantity, $-\beta$, is positive. Furthermore, when $\gamma$ goes to zero, we need $\zeta$ to go to zero as well. This will only happen if we adopt the inferior (negative) sign. Hence, the physically sensible root of this quadratic relation is given by the expression, $\zeta$ $=$ $\frac{1}{2\alpha }\{-\beta -[{\beta }^2-4\alpha \gamma {]}^{1/2}\}.$

Here we present a quantitatively accurate depiction of the shape of the (Ferrers) gravitational potential that arises from oblate-spheroidal configurations having a parabolic density distribution. We closely follow the discussion of equi-gravitational potential contours that arise in (uniform-density) Maclaurin spheroids. In order to facilitate comparison with Maclaurin spheroids, we will focus on a model with …

$\frac{a_s}{a_{\ell }}=0.582724,$	$e=0.81267,$
$A_{\ell }=A_m=0.51589042,$	$A_s=0.96821916,$	$I_{\mathrm{B}\mathrm{T}}=1.360556,$
$a_{\ell }^2{A}_{\ell \ell }=0.3287756,$	$a_{\ell }^2{A}_{ss}=1.5066848,$	$a_{\ell }^2{A}_{\ell s}=0.6848975.$

[NOTE:   Along the Maclaurin spheroid sequence, this is the eccentricity that marks bifurcation to the Jacobi ellipsoid sequence — see the first model listed in Table IV (p. 103) of [EFE] and/or see Tables 1 and 2 of our discussion of the Jacobi ellipsoid sequence. It is unlikely that this same eccentricity has a comparably special physical relevance along the sequence of spheroids having parabolic density distributions.]

The largest value of the gravitational potential that will arise inside (actually, on the surface) of the configuration is at $(\varpi ,z)=(1,0)$. That is, when,

${\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}{|}_{\mathrm{m}\mathrm{a}\mathrm{x}}$

$=$

$A_{\ell }-\frac{1}{2}{A}_{\ell \ell }{a}_{\ell }^2=0.3515026.$

So we will plot various equipotential surfaces having, $0<{\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}<{\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}{|}_{\mathrm{m}\mathrm{a}\mathrm{x}}$, recognizing that they will each cut through the equatorial plane $(z=0)$ at the radial coordinate given by,

${\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}$	$=$	$-\frac{1}{2}{A}_{ss}{a}_{\ell }^2{\xcancel{{\zeta }^2}}^0+[A_s-A_{\ell s}{a}_{\ell }^2(\frac{{\varpi }^2}{a_{\ell }^2}\left)\right]{\xcancel{\zeta }}^0+A_{\ell }\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)-\frac{1}{2}{A}_{\ell \ell }{a}_{\ell }^2\left(\frac{{\varpi }^4}{a_{\ell }^4}\right)$
$\Rightarrow 0$	$=$	$\frac{1}{2}{A}_{\ell \ell }{a}_{\ell }^2{\chi }^2-A_{\ell }\chi +{\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}},$

where,

$\chi \equiv \frac{{\varpi }^2}{a_{\ell }^2}.$

The solution to this quadratic equation gives,

${\chi }_{\mathrm{e}\mathrm{q}\mathrm{p}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{e}}$	$=$	$\frac{1}{A_{\ell \ell }{a}_{\ell }^2}\{A_{\ell }\pm [A_{\ell }^2-2A_{\ell \ell }{a}_{\ell }^2{\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}{]}^{1/2}\}$
	$=$	$\frac{A_{\ell }}{A_{\ell \ell }{a}_{\ell }^2}\{1-[1-\frac{2A_{\ell \ell }{a}_{\ell }^2{\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}}{A_{\ell }^2}{]}^{1/2}\}.$

Note that, again, the physically relevant root is obtained by adopting the inferior (negative) sign, as has been done in this last expression.

Equipotential Contours that Lie Entirely Within Configuration

For all $0<{\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}\le {\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}{|}_{\mathrm{m}\mathrm{i}\mathrm{d}}$, the equipotential contour will reside entirely within the configuration. In this case, for a given ${\varphi }_{\mathrm{c}\mathrm{h}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{e}}$, we can plot points along the contour by picking (equally spaced?) values of ${\chi }_{\mathrm{e}\mathrm{q}\mathrm{p}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{e}}\ge \chi \ge 0$, then solve the above quadratic equation for the corresponding value of $\zeta$.

In our example configuration, this means … (to be finished)

Hydrostatic Balance (Algebraic Condition)

Following our separate discussion of the equilibrium structure of Maclaurin spheroids, and given that our solution of the Poisson equation fixes the expression for ${\mathrm{\Phi }}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}$, the algebraic expression ensuring hydrostatic balance is,

$H(\varpi ,z)$

$=$

$C_B-[{\mathrm{\Phi }}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}(\varpi ,z)+\mathrm{\Psi }(\varpi ,z)],$

where, $\mathrm{\Psi }$ is the centrifugal potential. NOTE:   Generally when modeling axisymmetric astrophysical systems (see our accompanying discussion of simple rotation profiles) it is assumed that $\mathrm{\Psi }$ does not functionally depend on $z$. Here, our other constraints — for example, demanding that the configuration have a parabolic density distribution — may force us to adopt a $z$-dependent rotation profile.

Here, we know that the adopted parabolic density distribution gives rise to a gravitational potential of the form,

$\frac{{\mathrm{\Phi }}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}(\mathbf{𝐱})}{(-\pi G{\rho }_c{a}_{\ell }^2)}$

$=$

$\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}-\left[A_{\ell }\right(\frac{{\varpi }^2}{a_{\ell }^2})+A_s(\frac{z^2}{a_{\ell }^2}\left)\right]+\frac{1}{2}\left[A_{\ell \ell }{a}_{\ell }^2\right(\frac{{\varpi }^4}{a_{\ell }^4})+A_{ss}{a}_{\ell }^2(\frac{z^4}{a_{\ell }^4})+2A_{\ell s}{a}_{\ell }^2(\frac{{\varpi }^2{z}^2}{a_{\ell }^4}\left)\right].$

Hence,

$\mathrm{\Psi }(\varpi ,z)$	$=$	$C_B-{\mathrm{\Phi }}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}(\varpi ,z)-H(\varpi ,z)$
	$=$	$C_B+\pi G{\rho }_c{a}_{\ell }^2\{\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}-[A_{\ell }\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)+A_s\left(\frac{z^2}{a_{\ell }^2}\right)]+\frac{1}{2}[A_{\ell \ell }{a}_{\ell }^2\left(\frac{{\varpi }^4}{a_{\ell }^4}\right)+A_{ss}{a}_{\ell }^2\left(\frac{z^4}{a_{\ell }^4}\right)+2A_{\ell s}{a}_{\ell }^2\left(\frac{{\varpi }^2{z}^2}{a_{\ell }^4}\right)\left]\right\}-H_{c}h(\xi ).$

This can be rewritten in terms of T1 Coordinates. In particular, defining, $q\equiv a_{\ell }/a_s$ and, ${\xi }_1$ $\equiv$ $[z^2+(\frac{\varpi }{q}{)}^2{]}^{1/2}=a_s\left[\right(\frac{z}{a_s}{)}^2+(\frac{\varpi }{a_{\ell }}{)}^2{]}^{1/2}$ $\Rightarrow \frac{\rho }{{\rho }_c}$ $=$ $[1-(\frac{{\xi }_1}{a_s}{)}^2].$ Because we expect contours of constant enthalpy $(H)$ to coincide with contours of constant density in equilibrium configurations, we should expect to find that, $\frac{H}{H_c}$ $=$ $h({\xi }_1).$ If the "radial" enthalpy profile resembles our derived spherical enthalpy profile, we should expect to find that, $h({\xi }_1)$ $\sim$ $h_0[1-h_2{\xi }_1^2-h_4{\xi }_1^4]$ $\Rightarrow 1-\frac{h({\xi }_1)}{h_0}$ $\sim$ $h_2{\xi }_1^2+h_4{\xi }_1^4$   $=$ $h_2{a}_s^2\left[\right(\frac{z}{a_s}{)}^2+\left(\frac{\varpi }{a_{\ell }}{)}^2\right]+h_4\left\{a_s^2\right[(\frac{z}{a_s}{)}^2+(\frac{\varpi }{a_{\ell }}{)}^2]{\}}^2$   $=$ $h_2{a}_s^2\left[\right(\frac{z}{a_s}{)}^2+\left(\frac{\varpi }{a_{\ell }}{)}^2\right]+h_4{a}_s^4\left[\right(\frac{z}{a_s}{)}^4+2\left(\frac{z}{a_s}{)}^2\right(\frac{\varpi }{a_{\ell }}{)}^2+\left(\frac{\varpi }{a_{\ell }}{)}^4\right]$

Adopting this last expression for the enthalpy, we have,

$\frac{h({\xi }_1)}{h_0}$

$=$

$1-h_2{a}_s^2\left[\right(\frac{z}{a_s}{)}^2+\left(\frac{\varpi }{a_{\ell }}{)}^2\right]-h_4{a}_s^4\left[\right(\frac{z}{a_s}{)}^4+2\left(\frac{z}{a_s}{)}^2\right(\frac{\varpi }{a_{\ell }}{)}^2+\left(\frac{\varpi }{a_{\ell }}{)}^4\right].$

Hence,

$\mathrm{\Psi }(\varpi ,z)$	$=$	$C_B+\pi G{\rho }_c{a}_{\ell }^2\{\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}-[A_{\ell }\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)+A_s\left(\frac{z^2}{a_{\ell }^2}\right)]+\frac{1}{2}[A_{\ell \ell }{a}_{\ell }^2\left(\frac{{\varpi }^4}{a_{\ell }^4}\right)+A_{ss}{a}_{\ell }^2\left(\frac{z^4}{a_{\ell }^4}\right)+2A_{\ell s}{a}_{\ell }^2\left(\frac{{\varpi }^2{z}^2}{a_{\ell }^4}\right)\left]\right\}$
		$-H_c{h}_0\{1-h_2{a}_s^2[(\frac{z}{a_s}{)}^2+(\frac{\varpi }{a_{\ell }}{)}^2]-h_4{a}_s^4[(\frac{z}{a_s}{)}^4+2(\frac{z}{a_s}{)}^2(\frac{\varpi }{a_{\ell }}{)}^2+(\frac{\varpi }{a_{\ell }}{)}^4\left]\right\}$

---

At the pole configuration — that is, when $(\varpi ,z)=(0,a_s)$ — we want the enthalpy to go to zero. That means,

$\frac{C_B}{(-\pi G{\rho }_c{a}_{\ell }^2)}$	$=$	$[\frac{{\mathrm{\Phi }}_{\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{v}}(\varpi ,z)}{(-\pi G{\rho }_c{a}_{\ell }^2)}+\frac{\mathrm{\Psi }(\varpi ,z)}{(-\pi G{\rho }_c{a}_{\ell }^2)}{]}_{\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{e}}$
	$=$	$[\frac{\mathrm{\Psi }(\varpi ,z)}{(-\pi G{\rho }_c{a}_{\ell }^2)}{]}_{\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{e}}+\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}-[A_{\ell }{\xcancel{\left(\frac{{\varpi }^2}{a_{\ell }^2}\right)}}^0+A_s(1-e^2)]+\frac{1}{2}[A_{\ell \ell }{a}_{\ell }^2{\xcancel{\left(\frac{{\varpi }^4}{a_{\ell }^4}\right)}}^0+A_{ss}{a}_{\ell }^2(1-e^2{)}^2+2A_{\ell s}{a}_{\ell }^2{\xcancel{\left(\frac{{\varpi }^2{z}^2}{a_{\ell }^4}\right)}}^0]$
	$=$	$[\frac{\mathrm{\Psi }(\varpi ,z)}{(-\pi G{\rho }_c{a}_{\ell }^2)}{]}_{\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{e}}+\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}-[A_s(1-e^2)]+\frac{1}{2}[A_{ss}{a}_{\ell }^2(1-e^2{)}^2].$

For centrally condensed configurations, it is astrophysically reasonable to assume that $\mathrm{\Phi }(\varpi ,z)$ is of the form such that the centrifugal potential goes to zero when $\varpi \to 0$. Adopting that assumption here means,

$\frac{C_B}{(-\pi G{\rho }_c{a}_{\ell }^2)}$

$=$

$\frac{1}{2}{I}_{\mathrm{B}\mathrm{T}}-[A_s(1-e^2)]+\frac{1}{2}[A_{ss}{a}_{\ell }^2(1-e^2{)}^2].$

See Also