The Influence of Thermal Pressure on Equilibrium Models of Hypermassive Neutron Star Merger Remnants

TL;DR

This study examines how thermal pressure influences the stability and maximum mass of hypermassive neutron star merger remnants, revealing that thermal effects can extend their lifespan but do not significantly increase their maximum supported mass.

Contribution

It provides a detailed analysis of thermal pressure effects on equilibrium models of neutron star merger remnants using relativistic stellar structure equations and finite-temperature nuclear equations of state.

Findings

Hot maximum-mass configurations do not support larger baryonic masses than cold ones.

Thermal pressure significantly enhances mass support in subcritical spin configurations.

Thermal effects influence the stability and collapse timescales of merger remnants.

Abstract

The merger of two neutron stars leaves behind a rapidly spinning hypermassive object whose survival is believed to depend on the maximum mass supported by the nuclear equation of state, angular momentum redistribution by (magneto-)rotational instabilities, and spindown by gravitational waves. The high temperatures (~5-40 MeV) prevailing in the merger remnant may provide thermal pressure support that could increase its maximum mass and, thus, its life on a neutrino-cooling timescale. We investigate the role of thermal pressure support in hypermassive merger remnants by computing sequences of spherically-symmetric and axisymmetric uniformly and differentially rotating equilibrium solutions to the general-relativistic stellar structure equations. Using a set of finite-temperature nuclear equations of state, we find that hot maximum-mass critically spinning configurations generally do not support larger baryonic masses than their cold counterparts. However, subcritically spinning configurations with mean density of less than a few times nuclear saturation density yield a significantly thermally enhanced mass. Even without decreasing the maximum mass, cooling and other forms of energy loss can drive the remnant to an unstable state. We infer secular instability by identifying approximate energy turning points in equilibrium sequences of constant baryonic mass parametrized by maximum density. Energy loss carries the remnant along the direction of decreasing gravitational mass and higher density until instability triggers collapse. Since configurations with more thermal pressure support are less compact and thus begin their evolution at a lower maximum density, they remain stable for longer periods after merger.