User:Tohline/Apps/Ostriker64

Polytropic & Isothermal Tori

Overview

Here we will focus on the analysis of the structure self-gravitating tori that are composed of compressible — specifically, polytropic and isothermal — fluids as presented in a series of papers by Jeremiah P. Ostriker:

• J. Ostriker (1964, ApJ, 140, 1056) — The Equilibrium of Polytropic and Isothermal Cylinders
• J. Ostriker (1964, ApJ, 140, 1067) — The Equilibrium of Self-Gravitating Rings
• J. Ostriker (1964, ApJ, 140, 1529) — On the Oscillations and the Stability of a Homogeneous Compressible Cylinder
• J. Ostriker (1965, ApJ Supplements, 11, 167) — Cylindrical Emden and Associated Functions

I believe that much, if not all, of this material was drawn from Ostriker's doctoral dissertation research at the University of Chicago (and Yerkes Observatory) under the guidance of S. Chandrasekhar.

Coordinate System

In §IIa of Paper II, Ostriker defines a set of orthogonal coordinates, <math>~(r,\phi,\theta)</math>, that is related to the traditional Cartesian coordinate system, <math>~(x,y,z)</math>, via the relations,

<math>~x</math>	<math>~=</math>	<math>~(R+r\cos\phi)\cos\theta \, ,</math>
<math>~y</math>	<math>~=</math>	<math>~(R+r\cos\phi)\sin\theta \, ,</math>
<math>~z</math>	<math>~=</math>	<math>~r\sin\phi \, .</math>

As Ostriker states, "The coordinate <math>~r</math> is the distance from a reference circle of radius <math>~R</math> (later chosen to be the major radius of the ring) …" The angle, <math>~\theta</math>, plays the role of the azimuthal angle, as is familiar in both cylindrical and spherical coordinates, while, here, <math>~\phi</math> is a meridional-plane polar angle measured counterclockwise from the equatorial plane. For axisymmetric systems, there will be no dependence on the azimuthal angle, so the pair of relevant coordinates in the meridional plane are,

<math>~\varpi \equiv (x^2+y^2)^{1 / 2}</math>

<math>~=</math>

<math>~R+r\cos\phi \, ,</math>

and,

<math>~z</math>

<math>~=</math>

<math>~r\sin\phi \, .</math>

Figure 1 extracted without modification from p. 1077 of J. P. Ostriker (1964; Paper II) "The Equilibrium of Self-Gravitating Rings" ApJ, vol. 140, pp. 1067-1087 © American Astronomical Society

Second Attempt

Single Offset Circle

Now an off-center circle whose major and minor radii are, respectively, <math>~(\varpi_0,d)</math>, will be described by the expression,

<math>~d^2</math>

<math>~=</math>

<math>~ (\varpi - \varpi_0)^2 + z^2 \, . </math>

where both <math>~d</math> and <math>~\varpi_0</math> are held constant while mapping out the variation of <math>~z</math> with <math>~\varpi</math>. If we acknowledge that, in general, <math>~\varpi_0 \ne R_\mathrm{JPO}</math>, then we know how <math>~r</math> varies with <math>~\phi</math> via the relation,

<math>~d^2</math>	<math>~=</math>	<math>~ \biggl[ R_\mathrm{JPO} + r\cos\phi - \varpi_0\biggr]^2 + r^2\sin^2\phi </math>
	<math>~=</math>	<math>~ (R_\mathrm{JPO}-\varpi_0)^2 + 2\biggl[ (R_\mathrm{JPO}-\varpi_0) r\cos\phi \biggr] +r^2 </math>
<math>~\Rightarrow ~~~ 0 </math>	<math>~=</math>	<math>~ r^2 + 2r\biggl[ (R_\mathrm{JPO}-\varpi_0) \cos\phi \biggr] + \biggl[(R_\mathrm{JPO}-\varpi_0)^2 - d^2\biggr] </math>
<math>~\Rightarrow ~~~ r </math>	<math>~=</math>	<math>~ \frac{1}{2}\biggl\{ - 2\biggl[ (R_\mathrm{JPO}-\varpi_0) \cos\phi \biggr] \pm \sqrt{ 4\biggl[ (R_\mathrm{JPO}-\varpi_0) \cos\phi \biggr]^2 - 4\biggl[(R_\mathrm{JPO}-\varpi_0)^2 - d^2\biggr] } \biggr\} </math>
	<math>~=</math>	<math>~ \frac{1}{2}\biggl\{ 2\biggl[ (\varpi_0 - R_\mathrm{JPO}) \cos\phi \biggr] \pm \sqrt{ 4\biggl[ (\varpi_0 - R_\mathrm{JPO}) \cos\phi \biggr]^2 - 4\biggl[(\varpi_0 - R_\mathrm{JPO})^2 - d^2\biggr] } \biggr\} </math>
<math>~\Rightarrow~~~ \frac{r}{ (\varpi_0 - R_\mathrm{JPO}) }</math>	<math>~=</math>	<math>~ \cos\phi \pm \sqrt{ \cos^2\phi - 1 + d^2 (\varpi_0 - R_\mathrm{JPO})^{-2} } </math>
	<math>~=</math>	<math>~ \cos\phi \pm \sqrt{ d^2 (\varpi_0 - R_\mathrm{JPO})^{-2}-\sin^2\phi } </math>

In order to align this expression with the terminology (and variable labels) that we use in the context of a toroidal coordinate system, we associate the radius of the anchor ring as <math>~R_\mathrm{JPO}\leftrightarrow a</math>, and we associate the major radius of each circular torus as <math>~\varpi_0 \leftrightarrow R_0</math>. We therefore have,

<math>~\frac{r}{ (R_0-a) }</math>	<math>~=</math>	<math>~ \cos\phi \pm \sqrt{ d^2 (R_0-a)^{-2}-\sin^2\phi } </math>
<math>~\Rightarrow ~~~ \frac{r}{a}</math>	<math>~=</math>	<math>~\biggl(\frac{R_0}{a}-1 \biggr) \biggl[ \cos\phi \pm \sqrt{ \biggl(\frac{d}{a}\biggr)^2 \biggl(\frac{R_0}{a}-1 \biggr)^{-2}-\sin^2\phi } \biggr] </math>

and, the coordinates of points along the surface of the torus <math>~(\varpi,z)</math> are provided by the expressions,

<math>~\varpi</math>	<math>~=</math>	<math>~ a + (R_0 - a)\cos\phi \biggl[ \cos\phi \pm \sqrt{ d^2 (R_0 - a)^{-2}-\sin^2\phi } \biggr] </math>
<math>~z</math>	<math>~=</math>	<math>~ (R_0 - a)\sin\phi \biggl[ \cos\phi \pm \sqrt{ d^2 (R_0 - a)^{-2}-\sin^2\phi } \biggr] </math>

We have tested this pair of expressions using Excel and have successfully demonstrated that they do, indeed, trace out a circle of radius, <math>~d</math>, whose center is offset from the symmetry axis by a distance, <math>~R_0</math>.

Set of Circles Whose Offset Increases With Circle Diameter

A set of nested off-center circles will be described by allowing <math>~R_0 = R_0(d)</math>, that is, by having the off-set distance, <math>~R_0</math>, vary with the size of the circle, <math>~d</math>. The above prescription for the normalized "coordinate" <math>~r/a</math> will work for any prescribed <math>~R_0(d)</math> function.

But a particular <math>~R_0(d)</math> function is demanded if we want this derived prescription to represent the behavior of toroidal coordinates. In a toroidal coordinate system, a specification of the value of the "radial" coordinate, <math>~\eta</math>, automatically dictates the ratio <math>~R_0/d</math>; but we are not at liberty to separately define the value of the difference, <math>~(R_0 - d)</math>. Instead, we must enforce the toroidal-coordinate relation,

<math>~a^2</math>	<math>~=</math>	<math>~R_0^2 - d^2</math>
<math>~\Rightarrow~~~ \frac{R_0}{a}-1</math>	<math>~=</math>	<math>~\biggl[ 1 + \delta^2\biggr]^{1 / 2} -1 \, ,</math>

where we have adopted the shorthand notation, <math>~\delta\equiv d/a</math>. Hence,

<math>~\frac{r}{a}</math>

<math>~=</math>

<math>~[ \sqrt{1+\delta^2} -1 ] \{ \cos\phi \pm [\delta^2 ( \sqrt{1+\delta^2} -1 )^{-2}-\sin^2\phi ]^{1 / 2} \} </math>

Now, in a toroidal coordinate system, there is a similar "radial" coordinate, <math>~\eta</math>, whose value varies with distance from the anchor ring of radius, <math>~a</math>. Its value depends on both <math>~R_0</math> and <math>~d</math> via the relation,

<math>~R_0 = d\cosh\eta \, .</math>

This means that,

<math>~\cosh\eta</math>	<math>~=</math>	<math>~\frac{1}{\delta}\biggl(\frac{R_0}{a}\biggr) = \frac{\sqrt{1+\delta^2}}{\delta} </math>
<math>~\Rightarrow~~~ \delta^2 \cosh^2\eta</math>	<math>~=</math>	<math>~1 + \delta^2</math>
<math>~\Rightarrow~~~ \delta^2 </math>	<math>~=</math>	<math>~\frac{1}{\cosh^2\eta - 1} = \frac{1}{\sinh^2\eta} </math>
<math>~\Rightarrow~~~ \sqrt{1 + \delta^2} </math>	<math>~=</math>	<math>~\biggl[1 + \frac{1}{\sinh^2\eta} \biggr]^{1 / 2} = \coth\eta \, ,</math>

which also means that,

<math>~\frac{r}{a}</math>

<math>~=</math>

<math>~[ \coth\eta -1 ] \biggl\{ \cos\phi \pm \biggl[ ( \cosh\eta -\sinh\eta )^{-2} -\sin^2\phi \biggr]^{1 / 2} \biggr\} \, . </math>

Case of Small Offset

Another way to look at this issue is to go back to the expression,

<math>~d^2</math>	<math>~=</math>	<math>~ (R_\mathrm{JPO}-\varpi_0)^2 + 2\biggl[ (R_\mathrm{JPO}-\varpi_0) r\cos\phi \biggr] +r^2 </math>
<math>~\Rightarrow ~~~ \delta^2</math>	<math>~=</math>	<math>~\biggl(\frac{r}{a}\biggr)^2 + \frac{r}{a}\biggl[ 2\biggl(1 - \frac{R_0}{a}\biggr)\biggr] \cos\phi + \biggl(1 - \frac{R_0}{a}\biggr)^2 </math>

and assume that, while still dependent on the radial coordinate, the dimensionless offset is small. That is, assume that,

<math>~\Delta(\delta) \equiv 1 - \frac{R_0(\delta)}{a} \ll 1 \, .</math>

In this case, we can write,

<math>~ \delta^2</math>

<math>~\approx</math>

<math>~\biggl(\frac{r}{a}\biggr)^2 + 2\Delta(\delta) \biggl( \frac{r}{a} \biggr) \cos\phi +\cancelto{0}{\Delta^2(\delta)} \, . </math>

And differentiating both sides of the expression with respect to <math>~r/a</math> gives,

<math>~0 </math>

<math>~\approx</math>

<math>~2\biggl(\frac{r}{a}\biggr) + 2\Delta(\delta) \cos\phi</math>

First Attempt

Based on my (initial, casual) study of this paper, Figure 1 appears to illustrate a configuration in which the density is constant on various nested toroidal surfaces such that, working from the highest density location, outward, the location <math>~R</math> of the center of each torus shifts to larger and larger values. It would therefore appear as though Ostriker's <math>~R</math> must be a function of the density-marker. Using the subscript, <math>~i</math>, as the marker, we represent the density as a function, <math>~\rho(r_i)</math>, and recognize that <math>~R = R(r_i)</math> as well.

We recognize that the radial coordinate, <math>~\eta</math>, in a toroidal-coordinate system behaves in this same manner. Each <math>~\eta = ~ \mathrm{const}</math> surface is a circle of radius, <math>~d</math>, whose center is located a distance from the symmetry axis, <math>~R_0 = \sqrt{a^2 + d^2}</math>. And, holding <math>~a</math> fixed, the accompanying definition is,

<math>~\cosh\eta = \frac{R_0}{d} =\biggl[ 1 + \frac{a^2}{d^2} \biggr]^{1 / 2} = \frac{1}{\delta}\biggl[1 + \delta^{2} \biggr]^{1 / 2} \, ,</math>

where, <math>~\delta \equiv d/a</math>. Comparing Ostriker's notation with a toroidal coordinate system whose anchor ring is at the meridional-plane location <math>~(\varpi,z) = (a,0)</math>, we find that,

<math>~R+r\cos\phi</math>	<math>~=</math>	<math>~\frac{a\sinh\eta}{(\cosh\eta - \cos\theta)} \, ,</math>      and,
<math>~r\sin\phi</math>	<math>~=</math>	<math>~\frac{a\sin\theta}{(\cosh\eta - \cos\theta)} \, .</math>

It appears that we can make the following direct associations:   <math>~R_0 \leftrightarrow R_\mathrm{JPO}</math> and <math>~d \leftrightarrow r_\mathrm{JPO}</math>. Hence, we have,

<math>~\frac{d\sin\phi}{R_0+d\cos\phi}</math>	<math>~=</math>	<math>~\frac{\sin\theta}{\sinh\eta}</math>
<math>~\Rightarrow ~~~\sin\theta</math>	<math>~=</math>	<math>~\frac{d \sinh\eta \sin\phi}{R_0+d\cos\phi} \, .</math>

And,

<math>~R_0+d\cos\phi</math>	<math>~=</math>	<math>~\frac{a\sinh\eta}{(\cosh\eta - \cos\theta)} </math>
<math>~\Rightarrow ~~~\cos\theta</math>	<math>~=</math>	<math>~\cosh\eta - \frac{a\sinh\eta}{R_0+d\cos\phi} \, .</math>

Putting these together we find that,

<math>~1 = \sin^2\theta + \cos^2\theta</math>	<math>~=</math>	<math>~ \biggl[\frac{d \sinh\eta \sin\phi}{R_0+d\cos\phi} \biggr]^2 + \biggl[ \cosh\eta - \frac{a\sinh\eta}{R_0+d\cos\phi} \biggr]^2 </math>
<math>~\Rightarrow ~~~ (R_0 + d\cos\phi)^2</math>	<math>~=</math>	<math>~ [d \sinh\eta \sin\phi]^2 + [ \cosh\eta(R_0 + d\cos\phi) - a\sinh\eta]^2 </math>
<math>~\Rightarrow ~~~ \biggl[ \frac{R_0}{d} + \cos\phi\biggr]^2</math>	<math>~=</math>	<math>~ [\sinh\eta \sin\phi]^2 + \biggl[ \cosh\eta \biggl(\frac{R_0}{d} + \cos\phi \biggr) - \frac{a}{d} \sinh\eta \biggr]^2 </math>

See Also

• T. Fukushima (2016, AJ, 152, article id. 35, 31 pp.) — Zonal Toroidal Harmonic Expansions of External Gravitational Fields for Ring-like Objects
• W.-T. Kim & S. Moon (2016, ApJ, 829, article id. 45, 22 pp.) — Equilibrium Sequences and Gravitational Instability of Rotating Isothermal Rings
• E. Y. Bannikova, V. G. Vakulik & V. M. Shulga (2011, MNRAS, 411, 557 - 564) — Gravitational Potential of a Homogeneous Circular Torus: a New Approach
• D. Petroff & S. Horatschek (2008, MNRAS, 389,156 - 172) — Uniformly Rotating Homogeneous and Polytropic Rings in Newtonian Gravity

The following quotes have been taken from Petroff & Horatschek (2008): §1:   "The problem of the self-gravitating ring captured the interest of such renowned scientists as Kowalewsky (1885), Poincaré (1885a,b,c) and Dyson (1892, 1893). Each of them tackled the problem of an axially symmetric, homogeneous ring in equilibrium by expanding it about the thin ring limit. In particular, Dyson provided a solution to fourth order in the parameter <math>~\sigma = a/b</math>, where <math>~a = r_t</math> provides a measure for the radius of the cross-section of the ring and <math>~b = \varpi_t</math> the distance of the cross-section's centre of mass from the axis of rotation." §7:   "In their work on homogeneous rings, Poincaré and Kowalewsky, whose results disagreed to first order, both had made mistakes as Dyson has shown. His result to fourth order is also erroneous as we point out in Appendix B."

• P. H. Chavanis (2006, International Journal of Modern Physics B, 20, 3113 - 3198) — Phase Transitions in Self-Gravitating Systems
• M. Ansorg, A. Kleinwächter & R. Meinel (2003, MNRAS, 339, 515) — Uniformly Rotating Axisymmetric Fluid Configurations Bifurcating from Highly Flattened Maclaurin Spheroids
• M. Lombardi & G. Bertin (2001, Astronomy & Astrophysics, 375, 1091 - 1099) — Boyle's Law and Gravitational Instability
• W. Kley (1996, MNRAS, 282, 234) — Maclurin Discs and Bifurcations to Rings
• J. W. Woodward, J. E. Tohline, & I. Hachisu (1994, ApJ, 420, 247 - 267) — The Stability of Thick, Self-Gravitating Disks in Protostellar Systems
• I. Bonnell & P. Bastien (1991, ApJ, 374, 610 - 622) — The Collapse of Cylindrical Isothermal and Polytropic Clouds with Rotation
• J. E. Tohline & I. Hachisu (1990, ApJ, 361, 394 - 407) — The Breakup of Self-Gravitating Rings, Tori, and Thick Accretion Disks
• F. Schmitz (1988, Astronomy & Astrophysics, 200, 127 - 134) — Equilibrium Structures of Differentially Rotating Self-Gravitating Gases
• P. Veugelen (1985, Astrophysics & Space Science, 109, 45 - 55) — Equilibrium Models of Differentially Rotating Polytropic Cylinders
• M. A. Abramowicz, A. Curir, A. Schwarzenberg-Czerny, & R. E. Wilson (1984, MNRAS, 208, 279 - 291) — Self-Gravity and the Global Structure of Accretion Discs
• P. Bastien (1983, Astronomy & Astrophysics, 119, 109 - 116) — Gravitational Collapse and Fragmentation of Isothermal, Non-Rotating, Cylindrical Clouds
• Y. Eriguchi & D. Sugimoto (1981, Progress of Theoretical Physics, 65, 1870 - 1875) — Another Equilibrium Sequence of Self-Gravitating and Rotating Incompressible Fluid
• J. E. Tohline (1980, ApJ, 236, 160 - 171) — Ring Formation in Rotating Protostellar Clouds
• T. Fukushima, Y. Eriguchi, D. Sugimoto, & G. S. Bisnovatyi-Kogan (1980, Progress of Theoretical Physics, 63, 1957 - 1970) — Concave Hamburger Equilibrium of Rotating Bodies
• J. Katz & D. Lynden-Bell (1978, MNRAS, 184, 709 - 712) — The Gravothermal Instability in Two Dimensions
• P. S. Marcus, W. H. Press, & S. A. Teukolsky (1977, ApJ, 214, 584- 597) — Stablest Shapes for an Axisymmetric Body of Gravitating, Incompressible Fluid (includes torus with non-uniform rotation)

1. Shortly after their equation (3.2), Marcus, Press & Teukolsky make the following statement: "… we know that an equilibrium incompressible configuration must rotate uniformly on cylinders (the famous "Poincaré-Wavre" theorem, cf. Tassoul 1977, &Sect;4.3) …"

• C. J. Hansen, M. L. Aizenman, & R. L. Ross (1976, ApJ, 207, 736 - 744) — The Equilibrium and Stability of Uniformly Rotating, Isothermal Gas Cylinders
• C.-Y. Wong (1974, ApJ, 190, 675 - 694) — Toroidal Figures of Equilibrium
• C.-Y. Wong (1973, Annals of Physics, 77, 279 - 353) — Toroidal and Spherical Bubble Nuclei
• Gunnar Randers (1942, ApJ, 95, 88) — The Equilibrium and Stability of Ring-Shaped 'barred SPIRALS'.
• William Duncan MacMillan (1958), The Theory of the Potential, New York: Dover Publications
• Oliver Dimon Kellogg (1929), Foundations of Potential Theory, Berlin: Verlag Von Julius Springer
• Lord Rayleigh (1917, Proc. Royal Society of London. Series A, 93, 148-154) — On the Dynamics of Revolving Fluids
• F. W. Dyson (1893, Philosophical Transaction of the Royal Society London. A., 184, 1041 - 1106) — The Potential of an Anchor Ring. Part II.
  1. In this paper, Dyson derives the gravitational potential inside the ring mass distribution
• F. W. Dyson (1893, Philosophical Transaction of the Royal Society London. A., 184, 43 - 95) — The Potential of an Anchor Ring. Part I.
  1. In this paper, Dyson derives the gravitational potential exterior to the ring mass distribution
• S. Kowalewsky (1885, Astronomische Nachrichten, 111, 37) — Zusätze und Bemerkungen zu Laplace's Untersuchung über die Gestalt der Saturnsringe
• Poincaré (1885a, C. R. Acad. Sci., 100, 346), (1885b, Bull. Astr., 2, 109), (1885c, Bull. Astr. 2, 405). — references copied from paper by Wong (1974)

 

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