Equation of state for hot hyperonic neutron star matter

TL;DR

This paper extends the FSU2H equation-of-state model to finite temperature, analyzing its effects on hyperonic neutron star matter and challenging the common use of the $\Gamma$ law in simulations.

Contribution

It introduces a finite temperature extension of the FSU2H model and provides simple parameterizations for the thermal index in hyperonic matter.

Findings

Temperature effects are stronger with hyperons.

The $\Gamma$ law is inadequate for hyperonic matter.

Parameterizations enable easier access to finite temperature EOS.

Abstract

The FSU2H equation-of-state model, originally developed to describe cold neutron star matter with hyperonic cores, is extended to finite temperature. Results are presented for a wide range of temperatures and lepton fractions, which cover the conditions met in protoneutron star matter, neutron star mergers and supernova explosions. It is found that the temperature effects on the thermodynamical observables and the composition of the neutron star core are stronger when the hyperonic degrees of freedom are considered. An evaluation of the temperature and density dependence of the thermal index leads to the observation that the so-called $\Gamma$ law, widely used in neutron star merger simulations, is not appropriate to reproduce the true thermal effects, specially when hyperons start to be abundant in the neutron star core. To make finite temperature equations of state easily accessible, simple parameterizations of the thermal index for nucleonic and hyperonic $\beta$-stable neutrino-free matter are provided.