The dynamical mass ejection from binary neutron star mergers: Radiation-hydrodynamics study in general relativity

TL;DR

This study uses advanced radiation-hydrodynamics simulations in general relativity to analyze how different neutron star equations of state affect the properties of matter ejected during binary neutron star mergers, with implications for r-process element production.

Contribution

It introduces the first neutrino-inclusive radiation-hydrodynamics simulations of neutron star mergers using modern EOSs in numerical relativity.

Findings

Ejecta mass exceeds 0.01 solar masses only for soft EOS (SFHo).

Ejecta electron fraction distribution broadens due to shock heating.

Ejecta properties vary significantly with EOS, affecting r-process nucleosynthesis.

Abstract

We perform radiation-hydrodynamics simulations of binary neutron star mergers in numerical relativity on the Japanese "K" supercomputer, taking into account neutrino cooling and heating by an updated leakage-plus-transfer scheme for the first time. Neutron stars are modeled by three modern finite-temperature equations of state (EOS) developed by Hempel and his collaborators. We find that the properties of the dynamical ejecta of the merger such as total mass, average electron fraction, and thermal energy depend strongly on the EOS. Only for a soft EOS (the so-called SFHo), the ejecta mass exceeds $0.01M_{\odot}$. In this case, the distribution of the electron fraction of the ejecta becomes broad due to the shock heating during the merger. These properties are well-suited for the production of the solar-like $r$-process abundance. For the other stiff EOS (DD2 and TM1), for which a long-lived massive neutron star is formed after the merger, the ejecta mass is smaller than $0.01M_{\odot}$, although broad electron-fraction distributions are achieved by the positron capture and the neutrino heating.