Relativistic neutron star merger simulations with non-zero temperature equations of state I. Variation of binary parameters and equation of state

TL;DR

This study conducts neutron star merger simulations with various equations of state and binary parameters to understand how these factors influence the merger dynamics, remnant stability, and matter ejection.

Contribution

It systematically investigates the effects of different non-zero temperature EoSs, binary mass ratios, and spin states on merger outcomes using an approximate general relativity approach.

Findings

Remnant collapse depends on the EoS, with softer EoSs leading to immediate or short-term collapse.

Larger torus masses are associated with asymmetric systems, larger initial NSs, and higher total angular momentum.

Temperature in the post-merger torus ranges from 3 to 10 MeV, with some matter becoming gravitationally unbound.

Abstract

An extended set of binary neutron star (NS) merger simulations is performed with an approximative conformally flat treatment of general relativity to systematically investigate the influence of the nuclear equation of state (EoS), the neutron star masses, and the NS spin states prior to merging. We employ the two non-zero temperature EoSs of Shen et al. (1998a,b) and Lattimer & Swesty (1991). In addition, we use the cold EoS of Akmal et al. (1998) with a simple ideal-gas-like extension according to Shibata & Taniguchi (2006), and an ideal-gas EoS with parameters fitted to the supernuclear part of the Shen-EoS. We estimate the mass sitting in a dilute high-angular momentum ``torus'' around the future black hole (BH). The dynamics and outcome of the models is found to depend strongly on the EoS and on the binary parameters. Larger torus masses are found for asymmetric systems (up to ~0.3 M_sun for a mass ratio of 0.55), for large initial NSs, and for a NS spin state which corresponds to a larger total angular momentum. We find that the postmerger remnant collapses either immediately or after a short time when employing the soft EoS of Lattimer& Swesty, whereas no sign of post-merging collapse is found within tens of dynamical timescales for all other EoSs used. The typical temperatures in the torus are found to be about 3-10 MeV depending on the strength of the shear motion at the collision interface between the NSs and thus depending on the initial NS spins. About 10^{-3}-10^{-2} M_sun of NS matter become gravitationally unbound during or right after the merging process. This matter consists of a hot/high-entropy component from the collision interface and (only in case of asymmetric systems) of a cool/low-entropy component from the spiral arm tips. (abridged)