Onset of a Barmode Instability in Rotating, Initially Axisymmetric Polytropes

 "… the onset of instability is not very sensitive to the compressibility or angular momentum distribution of the polytrope when the models are parameterized by T/|W| — [in particular, the m = 2 barmode becomes unstable at T/|W| ∼ 0.26 - 0.28. ] The polytrope eigenfunctions are … qualitatively different from the Maclaurin eigenfunctions in one respect: they develop strong spiral arms. The spiral arms are stronger for more compressible polytropes and for polytropes whose angular momentum distributions deviate significantly from those of the Maclaurin spheroids." — Drawn from Toman, Imamura, Pickett & Durisen (1998), ApJ, 497, 370

Index of Relevant Publications

Here is a list of relevant research papers as largely enumerated by Y. Kojima & M. Saijo (2008)

 HYDROCODE: Newtonian, 3D Eulerian, 1st-order donor-cell on a cylindrical grid; π-symmetry plus reflection symmetry through equatorial plane; $~\Gamma=5/3$; Poisson solved with FFT + Buneman cyclic reduction MODEL(s): Constructed using Ostriker-Mark SCF method; axisymmetric, n = 3/2 polytrope; n' = 0 rotation law; four different equilibrium configurations having T/|W| = 0.28, 0.30, 0.33, 0.35.
 HYDROCODE: Same as in Tohline, Durisen & McCollough (1985), above. MODEL(s): Constructed using Ostriker-Mark SCF method; axisymmetric, n' = 0 rotation law; five different equilibrium configurations having (see column 1 of their Table 1) n = 0.8, 1.0, 1.3, 1.5, 1.8, all having T/|W| = 0.310.
 Two of the models that were studied in Williams & Tohline (1987) — specifically, the models having n = 0.8 and 1.8 — … are shown as they evolve to extremely nonlinear amplitudes: the end result in both cases is … The models shed a fraction of their mass and angular momentum, producing a ring which surrounds a more centrally condensed object … The central object is a triaxial figure that is rotating about its shortest axis.
 Using Newtonian dynamics and Newtonian gravity … we have carried out computer simulations of a differentially rotating compact star with a polytropic equation of state undergoing the dynamical bar instability. This instability has previously been modeled numerically by Tohline and collaborators in the context of star formation … Our work is the first to calculate [using a post-Newtonian approximation] the gravitational radiation produced by this instability, including wave forms and luminosities. It is also a significant advance over the earlier studies because, in addition to using better numerical techniques, we model the fluid correctly using an energy equation. This is essential due to the generation of entropy by shocks during the later stages of the evolution. HYDROCODE: Newtonian, 3D Lagrangian-Cartesian (SPH as implemented by Hernquist & Katz 1989), Includes energy equation and does not impose π-symmetry; $~\Gamma=5/3$; Poisson solved with TREESPH MODEL(s): Constructed using the SCF method of Smith & Centrella (1992; see article #15 in R. d'Inverno), which is based on earlier work of Ostriker-Mark 1968 and Hachisu 1986; axisymmetric, n = 3/2 polytrope; n' = 0 rotation law; only one equilibrium configuration, with T/|W| = 0.30.
 Extending the work of Houser, Centrella & Smith (1994) just mentioned — and closely paralleling the work of Williams & Tohline (1987, 1988) — they examine the development of the dynamical bar instability in configurations that have a range of different equations of state. HYDROCODE: Same as Houser, Centrella & Smith (1994) MODEL(s): Axisymmetric, n' = 0 rotation law; a total of 3 models examined with, respectively, (n, T/|W|) = (0.5, 0.31), (1.0, 0.32), (1.5, 0.32) — see their Table II. The SCF-based method used to construct these models and the (rather contrived) method used to map the continuum distribution of mass into the Lagrangian-based SPH code is discussed in detail in §III of the paper.

Break 1

Equilibrium Models constucted and examined by
$~n$ $~n'$ $~T/|W|$ $~\Omega_p/\Omega_\mathrm{eq}$ $~R_p/R_\mathrm{eq}$
$~\tfrac{5}{4}$ 0 0.274 2.44 0.258
0.279 2.45 0.250
0.288 2.49 0.234
0.319 2.65 0.187
0.346 2.88 0.147
0.363 3.11 0.123
$~\tfrac{3}{2}$ 0.266 2.97 0.258
0.280 3.03 0.234
0.290 3.09 0.218
0.300 3.16 0.202
0.316 3.28 0.179
0.327 3.35 0.163
0.344 3.61 0.139
0.368 4.04 0.107
0.396 4.79 0.0754
1 0.262 6.85 0.139
0.283 8.90 0.107
0.292 9.81 0.0958
0.311 12.4   0.0754
$~\tfrac{3}{2}$ 0.272 15.1   0.0754
0.280 16.9   0.0675
0.290 19.0   0.0595
$~\tfrac{5}{2}$ 0 0.262 7.19 0.202
0.268 7.26 0.194
0.273 7.34 0.187
0.285 7.52 0.171
0.297 7.73 0.155
0.304 7.85 0.147
0.324 8.29 0.123
0.338 8.68 0.107
0.371 9.85 0.0754
0.389 10.8   0.0595

Break 2

 Nonlinear growth of the bar-mode deformation is studied for a differentially rotating star with supercritical rotational energy. In particular, the growth mechanism of some azimuthal modes with odd wave numbers is examined … Mode coupling to even modes, i.e., the bar mode and higher harmonics, significantly enhances the amplitudes of odd modes … HYDROCODE: Newtonian, 3D Eulerian, Cartesian, with entropy tracer; reflection symmetry through equatorial plane; $~\Gamma=2$; Poisson solved with preconditioned conjugate gradient (PCG) method MODEL(s): axisymmetric, n = 1 polytrope; j-constant rotation law with A = 1; their Table I lists four different equilibrium configurations having T/|W| = 0.256, 0.268, 0.277, 0.281.

Additional references identified through the above set of references:

• M. Saijo (2018), Phys. Rev. D, 98, 024003: Determining the stiffness of the equation of state using low T/W dynamical instabilities in differentially rotating stars
 We investigate the nature of low T/W dynamical instabilities in various ranges of the stiffness of the equation of state in differentially rotating stars … We analyze these instabilities in both a linear perturbation analysis and a three-dimensional hydrodynamical simulation … the nature of the eigenfunction that oscillates between corotation and the surface for an unstable star requires reinterpretation of pulsation modes in differentially rotating stars.