Supramassive Neutron Stars


Stability and Collapse of Rapidly Rotating, Supramassive Neutron Stars

University of Illinois at Urbana-Champaign

Rotation can support stars with higher mass than the maximum limit for nonrotating, spherical stars, producing "supramassive" stars. 3D numerical simulations in full general relativity were performed to study the stability against collapse of rapidly rotating, supramassive neutron stars at the mass-shedding limit. The cases seen here are uniformly rotating. In the first case, the star is stable against collapse. In the second case, an unstable star collapses to a black hole in about a rotation period.

Equilibrium Sequence

This graph shows the gravitational mass of an equilibrium star as a function of the central density. All stars are supported by a polytropic equation of state with adiabatic index Gamma = 2. The curves denote the relations for nonrotating spherical neutron stars (dashed line) and stars rotating uniformly at the mass-shedding limit (solid line). Results from the numerical simulations showed that the onset of dynamical instability along the mass-shedding sequence nearly coincides with the onset of secular instability. The two dots represent the stable and unstable models evolved below.

Scene 1: Evolution of Stable Star

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This scene shows the density profile in the equatorial plane for a stable rotating star at the mass-shedding limit as it is evolved. The equatorial radius of the star is 5.1M and the polar radius is 3.4M. The rotational energy divided by the absolute value of the gravitational energy is T / |W| = 0.087. The color of the star indicates the rest-mass density. The arrow denotes the initial central density, which remains nearly constant in time.

Scene 2 - Part 1: Catastrophic Collapse of Unstable Star to a Black Hole

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This scene shows the density profile in the equatorial plane for an unstable rotating star as it collapses to a black hole. The equatorial radius is 4.2M and the polar radius is 3.2M. Here T / |W| = 0.080. An apparent horizon appears at the end of the collapse, denoting the formation of a Kerr black hole. The area of the horizon is A/AKerr = 1.05, close to the exact Kerr value. The angular momentum J/M2 = 0.56, appreciable but below the Kerr limit.

Scene 2 - Part 2: Lapse Profile of Collapsing Star

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Here, we see the evolution of the lapse function alpha as the star collapses to a black hole. The log of the lapse is plotted in the equatorial plane. The initial central lapse is 0.44. The final central lapse is 0.015. Its rapid plunge indicates the formation of a black hole.

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University of Illinois at Urbana-Champaign
last updated 5 Nov 14 aakhan3
Center for Theoretical Astrophysics---University of Illinois at Urbana-Champaign

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