Stability of hypermassive neutron stars with realistic rotation and entropy profiles

TL;DR

This paper investigates the stability of hypermassive neutron stars with realistic rotation and entropy profiles, confirming the turning point criterion for stability and exploring the effects of non-monotonic rotation and entropy distributions.

Contribution

It extends stability analysis to more realistic non-monotonic rotation and entropy profiles using 2D evolutions, confirming the turning point criterion in these cases.

Findings

Confirmed the turning point stability criterion for complex profiles.

Identified the rotation law parameters that maximize mass for given angular momentum.

Validated the new axisymmetric metric evolution capability of the Spectral Einstein Code.

Abstract

Binary neutron star mergers produce massive, hot, rapidly differentially rotating neutron star remnants; electromagnetic and gravitational wave signals associated with the subsequent evolution depend on the stability of these remnants. Stability of relativistic stars has previously been studied for uniform rotation and for a class of differential rotation with monotonic angular velocity profiles. Stability of those equilibria to axisymmetric perturbations was found to respect a turning point criterion: along a constant angular momentum sequence, the onset of unstable stars is found at maximum density less than but close to the density of maximum mass. In this paper, we test this turning point criterion for non-monotonic angular velocity profiles and non-isentropic entropy profiles, both chosen to more realistically model post-merger equilibria. Stability is assessed by evolving perturbed equilibria in 2D using the Spectral Einstein Code. We present tests of the code's new capability for axisymmetric metric evolution. We confirm the turning point theorem and determine the region of our rotation law parameter space that provides highest maximum mass for a given angular momentum.