Realistic models of general-relativistic differentially rotating stars

TL;DR

This paper develops realistic models of differentially rotating neutron star remnants post-merger, deriving new rotation laws from simulations and revealing unexpected stability properties related to angular momentum distribution.

Contribution

It introduces new differential rotation laws based on simulation data, improving the modeling of neutron star merger remnants in general relativity.

Findings

Traditional stability criteria may not apply to realistic remnants.

Remnants can have positive stability slope, indicating high angular momentum at large radii.

New rotation laws better match the properties of merger remnants.

Abstract

General-relativistic equilibria of differentially rotating stars are expected in a number of astrophysical scenarios, from core-collapse supernovae to the remnant of binary neutron-star mergers. The latter, in particular, have been the subject of extensive studies where they were modeled with a variety of laws of differential rotation with varying degree of realism. Starting from accurate and fully general-relativistic simulations of binary neutron-star mergers with various equations of state and mass ratios, we establish the time when the merger remnant has reached a quasi-stationary equilibrium and extract in this way realistic profiles of differential rotation. This allows us to explore how well traditional laws reproduce such differential-rotation properties and to derive new laws of differential rotation that better match the numerical data in the low-density Keplerian regions of the remnant. In this way, we have obtained a novel and somewhat surprising result: the dynamical stability line to quasi-radial oscillations computed from the turning-point criterion can have a slope that is not necessarily negative with respect to the central rest-mass density, as previously found with traditional differential-rotation laws. Indeed, for stellar models reproducing well the properties of the merger remnants, the slope is actually positive, thus reflecting remnants with angular momentum at large distances from the rotation axis, and hence with cores having higher central rest-mass densities and slower rotation rates.