Appendix/Ramblings/BdHN

Binary-driven Hpernovae

The material presented here builds on our separate discussion of close binary stars.

Setup

Initial Carbon-Oxygen Core

The total angular momentum associated with a Maclaurin spheroid of eccentricity, $e$, is given by the expression,

$L_{*}^2\equiv \frac{L^2}{(G{M}^3\overline{a})}$	$=$	$\frac{6}{5^2}[(3-2e^2)(1-e^2{)}^{1/2}\cdot \frac{{\sin }^{-1}e}{e^3}-\frac{3(1-e^2)}{e^2}](1-e^2{)}^{-2/3}.$
📚 Marcus, Press, & Teukolsky (1977), §IVa, p. 591, Eq. (4.2)

According to an accompanying table …

Jacobi Sequence Bifurcation:

• $[e,{\omega }_0^2/(4\pi G\rho ),j^2]=[0.812670,0.0935574,0.00455473]\Rightarrow [e,{\omega }_0^2/(\pi G\rho ),L_{*}]=[0.812670,0.374230,0.303751]$

Dynamically Unstable 2^nd Harmonic:

• $[e,{\omega }_0^2/(4\pi G\rho ),j^2]=[0.95289,0.11006,0.012796]\Rightarrow [e,{\omega }_0^2/(\pi G\rho ),L_{*}]=[0.95289,0.44024,0.25921]$

Now, given that, ${\omega }_0^2=(2\pi /P{)}^2$, if we specify $P$, then we know ${\rho }_{\mathrm{C}\mathrm{O}}$ from the expression,

${\rho }_{\mathrm{C}\mathrm{O}}$

$=$

$(\frac{{\omega }_0^2}{\pi G\rho }{)}^{-1}\frac{(2\pi /P{)}^2}{\pi G};$

and if we specify $M_{\mathrm{C}\mathrm{O}}$, we know ${\overline{a}}_{\mathrm{C}\mathrm{O}}$ via the expression,

${\overline{a}}_{\mathrm{C}\mathrm{O}}$

$=$

$(\frac{3M_{\mathrm{C}\mathrm{O}}}{4\pi {\rho }_{\mathrm{C}\mathrm{O}}}{)}^{1/3}.$

Initial CO Core at the Jacobi Bifurcation Point with $M_{\mathrm{C}\mathrm{O}}=10M_{\odot }$ and various spin periods, $P$
$P$ (min)	$P$ (s)	${\rho }_{\mathrm{C}\mathrm{O}}=\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{1}/P^2$		${\overline{a}}_{\mathrm{C}\mathrm{O}}=\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{2}\cdot {\rho }_{\mathrm{C}\mathrm{O}}^{-1/3}$		$L_{\mathrm{C}\mathrm{O}}=\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{3}\cdot {\overline{a}}^{1/2}$
		factor1	(g cm-3)	factor2	(cm)	factor3	(g cm2 s-1)
100	6600	…	1.155 × 101	…	7.435 × 1010	…	6.001 × 1052
5	300	…	5.592 × 103	…	9.469 × 109	…	2.142 × 1052
$G=6.672\times 10^{-8}$ cm3 g-1 s-2 $M_{\odot }=1.989\times 10^{33}$ g $\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{1}=(0.37423{)}^{-1}\cdot \frac{4\pi }{G}=5.033\times 10^8$ cgs $\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{2}=1.681\times 10^{11}$ cgs $\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}\mathrm{3}=(0.303751)\cdot (G{M}_{\mathrm{C}\mathrm{O}}^3{)}^{1/2}=2.201\times 10^{47}$ cgs

Related Binary

Consider the simple model of two spherical stars in circular orbit about one another, as depicted here on the right. In addition to the physical parameters explicitly labeled in this diagram, we adopt the following variable notation:

The total system mass is, $M_{\mathrm{t}\mathrm{o}\mathrm{t}}\equiv M+M^{\text{'}};$ The ratio of the primary to secondary mass is, $\lambda \equiv \frac{M}{M^{\text{'}}};$ And the separation between the two centers is, $d\equiv r_{\mathrm{c}\mathrm{m}}+r_{\mathrm{c}\mathrm{m}}^{\text{'}}.$

For a circular orbit, the angular velocity is related to the the system mass and separation via the Kepler relation,

${\omega }^2{d}^3=G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}},$

and the distances, $r_{\mathrm{c}\mathrm{m}}$ and $r_{\mathrm{c}\mathrm{m}}^{\text{'}}$, between the center of each star and the center of mass (cm) of the system must be related to one another via the expression,

$\frac{r_{\mathrm{c}\mathrm{m}}^{\text{'}}}{r_{\mathrm{c}\mathrm{m}}}$

$=$

$\frac{M}{M^{\text{'}}}=\lambda .$

Note that the following relations also hold:

$M=M_{\mathrm{t}\mathrm{o}\mathrm{t}}\left(\frac{\lambda }{1+\lambda }\right)$	and	$M^{\text{'}}=M_{\mathrm{t}\mathrm{o}\mathrm{t}}\left(\frac{1}{1+\lambda }\right)$
$r_{\mathrm{c}\mathrm{m}}=d\left(\frac{1}{1+\lambda }\right);$	and	$r_{\mathrm{c}\mathrm{m}}^{\text{'}}=d\left(\frac{\lambda }{1+\lambda }\right).$

Hence, the orbital angular momentum is,

$L_{\mathrm{o}\mathrm{r}\mathrm{b}}$	$=$	$[M{r}_{\mathrm{c}\mathrm{m}}^2+M^{\text{'}}(r_{\mathrm{c}\mathrm{m}}^{\text{'}}{)}^2]\omega$
	$=$	$M_{\mathrm{t}\mathrm{o}\mathrm{t}}{d}^2\left[\right(\frac{\lambda }{1+\lambda }\left)\right(\frac{1}{1+\lambda }{)}^2+\left(\frac{1}{1+\lambda }\right)\left(\frac{\lambda }{1+\lambda }{)}^2\right][\frac{G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{d^3}{]}^{1/2}$
	$=$	$(G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}^{3}d{)}^{1/2}[\frac{\lambda }{(1+\lambda {)}^2}].$

Assuming that both stars are rotating synchronously with the orbit, their respective spin angular momenta are,

$L_M=I_M\omega$	$=$	$\frac{2}{5}M{R}^2\omega$
	$=$	$\frac{2}{5}{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}\left(\frac{\lambda }{1+\lambda }\right)R^2[\frac{G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{d^3}{]}^{1/2}$
	$=$	$\frac{2}{5}(G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}^{3}d{)}^{1/2}(\frac{\lambda }{1+\lambda }\left)\right(\frac{R}{d}{)}^2,$
$L_{M^{\text{'}}}=I_{M^{\text{'}}}\omega$	$=$	$\frac{2}{5}{M}^{\text{'}}(R^{\text{'}}{)}^2\omega$
	$=$	$\frac{2}{5}{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}\left(\frac{1}{1+\lambda }\right)(R^{\text{'}}{)}^2[\frac{G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{d^3}{]}^{1/2}$
	$=$	$\frac{2}{5}(G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}^{3}d{)}^{1/2}(\frac{1}{1+\lambda }\left)\right(\frac{R^{\text{'}}}{d}{)}^2.$

Hence, the total angular momentum of the system is,

$L_{\mathrm{t}\mathrm{o}\mathrm{t}}=L_{\mathrm{o}\mathrm{r}\mathrm{b}}+L_M+L_{M^{\text{'}}}$	$=$	$(G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}^{3}d{)}^{1/2}[\frac{\lambda }{(1+\lambda {)}^2}]+\frac{2}{5}(G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}^{3}d{)}^{1/2}\left(\frac{\lambda }{1+\lambda }\right)(\frac{R}{d}{)}^2$
		$+\frac{2}{5}(G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}^{3}d{)}^{1/2}(\frac{1}{1+\lambda }\left)\right(\frac{R^{\text{'}}}{d}{)}^2$
	$=$	$(G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}^{3}R{)}^{1/2}\{\left[\frac{\lambda }{(1+\lambda {)}^2}\right](\frac{d}{R}{)}^{1/2}+\frac{2}{5}(\frac{1}{1+\lambda }\left)\right[\lambda +\left(\frac{R^{\text{'}}}{R}{)}^2\right]\left(\frac{d}{R}{)}^{-3/2}\right\}$

Solutions

Here we assume that the unprimed parameters (primary star) refer to the Maclaurin spheroid component while the primed parameters (secondary star) refer to the Jacobi ellipsoid component. Also assume that the mass ratio is,

$\lambda \equiv \frac{M}{M^{\text{'}}}$

$=$

$\frac{M_{\mathrm{C}\mathrm{O}\mathrm{f}}}{M_{\mathrm{J}\mathrm{T}\mathrm{E}}}=\frac{8.5}{1.5}=\frac{17}{3}=5.6667.$

Assume that the total angular momentum and total mass are conserved, and that the three frequencies that appear in the binary system — orbital, and two stellar spins — are identical (co-rotation). This means that the orbital separation, $d$, can be expressed in terms of ${\omega }_f$ via the Keperian relation,

$d^3$

$=$

$\frac{G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{{\omega }_f^2}.$

And the radii of the binary components are given, respectively, by the relations,

$R^3$	$=$	$(c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}[\frac{\lambda G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{(1+\lambda ){\omega }_f^2}];$
$(R^{\text{'}}{)}^3$	$=$	$(c_2{)}_{\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{y}}[\frac{G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{(1+\lambda ){\omega }_f^2}].$

Hence, the length-ratios that appear in the expression for $L_{\mathrm{t}\mathrm{o}\mathrm{t}}$ may be re-expressed as follows:

$(\frac{R^{\text{'}}}{R}{)}^3$	$=$	$(c_2{)}_{\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{y}}[\frac{G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{(1+\lambda ){\omega }_f^2}\left]\right\{(c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}[\frac{\lambda G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{(1+\lambda ){\omega }_f^2}]{\}}^{-1}$
	$=$	$\frac{(c_2{)}_{\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{y}}}{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}};$
$(\frac{d}{R}{)}^3$	$=$	$\frac{G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{{\omega }_f^2}\{(c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}\left[\frac{\lambda G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{(1+\lambda ){\omega }_f^2}\right]{\}}^{-1}$
	$=$	$\frac{(1+\lambda )}{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}.$

Also,

$\frac{L_{\mathrm{t}\mathrm{o}\mathrm{t}}^6}{(G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}^{3}R{)}^3}=\frac{L_{*}^6(G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}^3{\overline{a}}_{\mathrm{C}\mathrm{O}}^3)}{(G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}^{3}R{)}^3}$	$=$	$L_{*}^6{\overline{a}}_{\mathrm{C}\mathrm{O}}^3\{(c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}\left[\frac{\lambda G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}{(1+\lambda ){\omega }_f^2}\right]{\}}^{-1}$
	$=$	$\frac{(1+\lambda )L_{*}^6}{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}\left[\frac{{\overline{a}}_{\mathrm{C}\mathrm{O}}^3{\omega }_f^2}{G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}\right].$

As a result, we find,

$\frac{(1+\lambda )L_{*}^6}{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}\left[\frac{{\overline{a}}_{\mathrm{C}\mathrm{O}}^3{\omega }_f^2}{G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}\right]$	$=$	$\left\{\right[\frac{\lambda }{(1+\lambda {)}^2}\left]\right(\frac{d}{R}{)}^{1/2}+\frac{2}{5}\left(\frac{1}{1+\lambda }\right)[\lambda +(\frac{R^{\text{'}}}{R}{)}^2\left]\right(\frac{d}{R}{)}^{-3/2}{\}}^6$
$\Rightarrow \left[\frac{{\overline{a}}_{\mathrm{C}\mathrm{O}}^3{\omega }_f^2}{G{M}_{\mathrm{t}\mathrm{o}\mathrm{t}}}\right]$	$=$	$\frac{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}{(1+\lambda )L_{*}^6}\left\{\right[\frac{\lambda }{(1+\lambda {)}^2}\left]\right[\frac{(1+\lambda )}{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}{]}^{1/6}+\frac{2}{5}\left(\frac{1}{1+\lambda }\right)[\lambda +(\frac{(c_2{)}_{\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{y}}}{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}{)}^{2/3}\left]\right[\frac{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}{(1+\lambda )}{]}^{1/2}{\}}^6$
$\Rightarrow {\omega }_f^2$	$=$	$\frac{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}{(1+\lambda )L_{*}^6}\left[\frac{3(c_0{)}_{\mathrm{C}\mathrm{O}}{P}^2}{(4\pi {)}^2}\right]\left\{\right[\frac{\lambda }{(1+\lambda {)}^2}\left]\right[\frac{(1+\lambda )}{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}{]}^{1/6}+\frac{2}{5}\left(\frac{1}{1+\lambda }\right)[\lambda +(\frac{(c_2{)}_{\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{y}}}{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}{)}^{2/3}\left]\right[\frac{\lambda (c_2{)}_{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{m}\mathrm{a}\mathrm{r}\mathrm{y}}}{(1+\lambda )}{]}^{1/2}{\}}^6.$

Critique

Manuscript Review

Overview

It is not yet well-established what type of stellar configuration serves as the precursor to long-GRBs. Zhang and Ruffini argue that if you have a CO_star in orbit about a NS and assume that the angular rotation frequency of the CO_star is synchronous with its orbital frequency, there are situations in which the (core of the?) CO_star will be spinning sufficiently fast that it spontaneously undergoes fission. The authors imagine that the result will be a compact binary within a compact binary, opening the door for a rather exotic subsequent (hypernova?) explosion; an explosion that exhibits an emission/radiation signature that is more complex and more drawn out in time than that of a "simple" hypernova.

At the heart of the authors' very speculative hypothesis is the suggestion that a CO_star can spontaneously undergo fission. The authors appear to be thinking that the classical fission theory of star formation is relevant to this problem. However, they present an inaccurate, as well as seriously incomplete, depiction of the classical fission theory [see our Notes 1A, 1B, and 1C, below].

Furthermore, the authors do not address serious questions that have been raised over the past 40+ years regarding the viability of the fission theory. Using numerical hydrodynamic techniques, various research groups have modeled the growth of non-axisymmetric structure in rapidly rotating protostellar clouds as well as in collapsing stellar cores. Nonlinear-amplitude bar-like and spiral-shaped structures develop, but fission into a binary system does not occur [see our Note 2, below].

Finally, the bulk of this manuscript (sections 2 and 3) is used by the authors to develop, then use, a toy (algebraic) model to illustrate what the properties might be of a binary system that results from the fission of a CO_star. As is explained below [see our Note 3], there is nothing new (astrophysically or mathematically) in this presentation. At best, this portion of the manuscript should be significantly compressed and earlier work properly referenced, then relegated to an appendix of a more substantive article.

NOTES:

[1] Is the classical "Fission Theory" relevant?

A. An excellent presentation regarding the classical "Fission Theory of Binary Stars" can be found in Lebovitz, N. R. (1972, ApJ, 175, p. 171) -- see especially the 2nd paragraph of section IV (pp. 176 - 177) where reference is made to the Jacobi bifurcation at e = 0.8127, and bifurcation to the "lower self-adjoint (LSA)" sequence at e = 0.9529. Both are points along the Maclaurin sequence where the the axisymmetric configuration is susceptible to deformation into a nonaxisymmetric (specifically, ellipsoidal) configuration: In the presence of viscous dissipation, the Jacobi bifurcation point is relevant; in the absence of viscosity, the LSA bifurcation point is relevant. In section 2 of their manuscript, Zhang and Ruffini focus on a "before fission" model that sits at the point along the Maclaurin sequence where the Jacobi sequence bifurcates. In the context of the precursor of long-GRBs, why have the authors focused on bifurcation to the Jacobi sequence instead of the LSA sequence?

B. In their effort to illustrate what the properties of an "after fission" binary system might be, Zhang and Ruffini pick: a system in which the mass ratio is 17/3; the more massive component is a Maclaurin spheroid; and the less massive component is a Jacobi ellipsoid. Why is this an appropriate "after fission" configuration? They seem to be suggesting that fission occurs precisely when the "before fission" CO_star evolves to the Jacobi bifurcation point and that it splits in such a way that one fission component lands on one equilibrium branch -- the Maclaurin sequence -- while the second lands on the other equilibrium branch -- the Jacobi sequence. This is quite different from the scenario that is suggested by the classical fission theory (see the following, note 1.C). What is the physical justification for the scenario being proposed by Ahang and Ruffini?

C. The classical fission theory is quantitatively well illustrated by Eriguchi, Y., and Hachisu, I. (1982, Progress of Theoretical Physics, 67, p. 844) -- see especially their Figure 1 -- and by Eriguchi, Y., Hachisu, I., and Sugimoto, D. (1982, Progress of Theoretical Physics, 67, p. 1068) -- see especially their Figs. 1, 3, and 4. Applying this classical theory to the physical scenario being investigated by Zhang and Ruffini, we would expect the following: After the axisymmetric CO_star encounters the Jacobi bifurcation point, the star should deform into an ellipsoidal configuration that becomes more and more elongated on a (slow) viscous timescale. Eventually, a point is encountered along the Jacobi sequence where a so-called dumbbell/binary sequence bifurcates from the Jacobi sequence (a_2, a_3) = (0.2972, 0.2575); it is at this point that the configuration becomes susceptible to fission into a binary system with a mass ratio of unity.

[2] Is the classical concept of fission viable in any physical context?

Over the past 40+ years, various modeling efforts have examined the onset and/or the nonlinear development of nonaxisymmetric structure in rapidly rotating configurations that are more realistic than the (incompressible) models considered by the classical fission hypothesis. All seem to indicate that the outcome is not fission. See, for example, Lebovitz, N. R. (1974, ApJ, 190, 121); or Tohline, J. E. (2002, Annual Review of Astronomy and Astrophysics, Vol. 40, 349) -- especially the section titled "Delayed Breakup" (pp. 367 - 374) -- or Ott, C. D. et al. (2005, ApJ, 625, L119).

As a case in point, their toy model "before fission" is a (10 solar-mass) Maclaurin spheroid; next, they envision that fission occurs when the initial CO_star is rotating sufficiently fast that its eccentricity places it at the point along the Maclaurin sequence where the Jacobi sequence bifurcates (e = 0.8127); finally, their toy model "after fission" is a (1.5 solar-mass) Jacobi ellipsoid paired with an 8.5 solar-mass Maclaurin spheroid; . This is inaccurate depiction of the classical fission theory [see note 1, below].

For example, in section 2 of the manuscript, their toy model of a "before fission" CO_star is a Maclaurin spheroid whose spin frequency places it at the point along the Maclaurin sequence where the Jacobi ellipsoid sequence bifurcates. Apparently they imagine that that configuration spontaneously fissions, because in section 3 the adopted "after fission" toy model is a binary system with the same mass and total angular momentum as the "before fission" configuration. This is not the correct interpretation of the classical fission hypothesis [see note 6, below]. Furthermore, to my knowledge, there is no evidence in the context of star formation studies that evolution actually leads to fission!

I am extremely skeptical that fission will Unfortunately, I am left unconvinced the authors the assertion that an initially axisymmetric CO_star can be spun up to the point where it encounters undergo fission.

Although tantalizing, this is a very speculative idea.

Unfortunately, the manuscript offered by Zhang and Ruffini offers very little astrophysical substance beyond the hand-waving argument found in their abstract -- or in the opening paragraph of this report. The authors provide absolutely no justification for fission. Indeed, although the model that is presented for illustration purposes in sections 2 and 3 of their manuscript is couched in terms of Maclaurin spheroids and Jacobi ellipsoids, the authors do not appear to understand how fission is envisioned to work in the classical model of (slow) evolution along the Maclaurin or Jacobi sequences. The authors also do not appear to be aware of published works over the past couple of decades in which hydrodynamic techniques have been used in an effort to simulate the process of fission in (e.g., compressible) models that are more astrophysically realistic than Maclaurin/Jacobi configurations; to the best of my knowledge, fission is never the outcome! Fission is a critical starting premise of the Zhang and Ruffini paper. Is there any evidence to support it?

The meat of this manuscript resides in sections 2 and 3. The authors illustrate, through a sequence of algebraic steps, how the physical parameters of the "after fission" binary system can be deduced from the (specified) parameters of the rapidly rotating "before fission" spheroidal configuration. This presentation is underwhelming. The relevant algebraic relations have appeared in various astrophysical publications for over 100 years -- see references, below, to Thomas & Tait (1867) and to Darwin (1906). By adopting relations from these earlier works, these two manuscript sections can be significantly reduced in length (and very likely combined); and, we would argue, Tables 3 and 4 should be omitted altogether. Given that there is nothing really new, here, we recommend that, at best, the meat of this manuscript should be relegated to the appendix of a paper that substantiates in a much more profound way why the core fission hypothesis.

Quantitative Model

The authors begin (section 2 of the manuscript) by supposing that (the pre-collapse core of) a CO star is represented by a Maclaurin spheroid, that is, represented by "configuration #1" as we have described above. They assume that the configuration's eccentricity, e = 0.812670; therefore its dimensionless spin frequency is 0.37423 (manuscript Eq. 1) and its dimensionless angular momentum is 0.30375 (manuscript Eq. 5). As their abstract indicates, their ultimate aim is to understand long GRBs, so they assume that the CO_star's mass is 10 solar masses and that its spin period, P, is somewhere between 5 minutes to 110 minutes (see Table 1 of the manuscript). Given these configuration specifications, they are able to determine the values of a variety of physical parameters (again, see Table 1); of particular interest is the configuration's total angular momentum, J_CO.

Next (section 3 of the manuscript), the authors ask what the final system would look like if the rotating CO_star were to undergo fission while conserving its total mass and total angular momentum. As they have envisioned it, the final state is essentially "configuration #2," as we have described above. They therefore **could** have used Darwin's (1906) expression for the total angular momentum of this configuration to determine the properties of the resulting binary-star system; and this could easily have been done for a wide variety of assumed values of key dimensionless parameters. Instead, they follow a more awkward path toward determining the values of the various physical parameters listed in their Table 2 while restricting their presentation to fixed values of: (a) the binary mass ratio (8.5/1.5 = 17/3); and (b) the square of the dimensionless spin frequency of both stars as stated in their Eqs. 10 and 13 (0.36316 and 0.373190, respectively).

Fission

The concept of fission as viewed in the context of classical studies of rotating, uniform-density equilibrium figures has been discussed in many publications over the past 100+ years. What appears to be most relevant in the context of the manuscript presently under review is the point along the Maclaurin spheroid sequence where bifurcation to the Jacobi sequence occurs. It occurs where the spheroid's eccentricity is, e = 0.812670.

Apparently -- although the authors do not explicitly explain why -- this point of bifurcation is important in the context of their fission scenario. We presume that this is the case because: (a) This is the eccentricity assigned by the authors to their initially axisymmetric CO_star at the onset of fission; and (b) in their scenario, fission results in the birth of two stars ... one is a (M_COf = 8.5 solar mass) configuration that sits at a different point along the Maclaurin sequence while the other is a (M_JTE = 1.5 solar mass) configuration that sits on the Jacobi ellipsoid sequence. Why do the authors think that fission occurs at the Jacobi bifurcation point? On what timescale -- dynamical or viscous? -- do they envision that the process of fission will occur; and why? Why aren't the resulting fission components identical?

Lebovitz, N. R. (1972, ApJ, 175, p. 171) -- See especially the 2nd paragraph of section IV (pp. 176 - 177). References both the Jacobi bifurcation at e = 0.8127, and bifurcation to the "lower self-adjoint" series at e = 0.9529.

Eriguchi, Y., and Hachisu, I. (1982, Progress of Theoretical Physics, 67, p. 844) -- See especially Figure 1. The point (e = 0.8127) at which the Jacobi ellipsoid sequence bifurcates from the Maclaurin spheroid sequence is labeled "B"; a point (a_2, a_3) = (0.2972, 0.2575) [Omega^2/(pi G rho) = 0.2128; L_* = 0.48242] at which the dumbbell-binary sequence bifurcates from the Jacobi sequence is labeled "I".

Eriguchi, Y., Hachisu, I., and Sugimoto, D. (1982, Progress of Theoretical Physics, 67, p. 1068) -- See especially Figs. 1, 3, and 4 where the classic route to fission is depicted quantitatively.

Overview

Start with a (10 solar-mass) "CO star" in orbit about a NS with orbital period, P = 2pi/Omega, and assume the CO star also spins with period P (synchronous). For the sake of simplicity, the authors treat the CO star as a rotationally flattened, uniform-density (Maclaurin) spheroid, in which case by simply **specifying** the spheroid's eccentricity, e_CO, they know the star's density and its average radius, and also ...

• Its dimensionless angular momentum; see, for example, Eq. (4.2) on p. 591 of [1];
• Its ratio of physically relevant time-scales, Omega^2/(pi G \rho); see, for example, Eq. (6) on p. 78 of [2], or much earlier, Eq. (1) on p. 613 of [3].

The numerical values -- 0.30375 and 0.37423, respectively -- that the authors have provided for these two physically interesting quantities (see their Eqs. 5 and 1, respectively) come from their assumption that e_CO = 0.812760.

Now assume that this CO star "fissions" into two component stars in orbit about one another, and that it does so while conserving total mass and total angular momentum.

Background

Let configuration #1 be a Maclaurin spheroid. Once the spheroid's eccentricity (e) has been specified, we know its dimensionless spin frequency [see note 1, below] and we know its dimensionless angular momentum [see note 2, below]. The configuration's spin frequency (Omega_0) and angular momentum (J_tot) can be expressed in physical units if, furthermore, the configuration's mass and spin period are specified.

Let configuration #2 be a binary system that contains a pair of uniform-density spheres in circular orbit about one another; assume furthermore that both spheres have a spin frequency that is the same as (is synchronized with) the orbital frequency (Omega_f). Darwin (1906) provides an expression for this system's dimensionless total angular momentum, in terms of the binary's mass ratio and the ratio of the radius of each star to the orbital separation [see note 3, below].

[1] See, for example, Eq. (6) on p. 78 of Chandrasekhar's EFE, or much earlier, Eq. (1) on p. 613 of Thomas & Tait (1867).

[2] See, for example, Eq. (4.2) on p. 591 of Marcus, Press, & Teukolsky (1977).

[3] See, for example, the expression for L_1 immediately following Eq. (1) on p. 165 of Darwin (1906).

Additional Minor Concerns

1. Ill-advised to refer to the new NS as "νNS" because, in this context, readers might reasonably associate the greek letter, ν, with neutrinos.

See Also