Dynamical Mass Ejection from Binary Neutron Star Mergers

TL;DR

This paper presents detailed simulations of neutron star mergers, analyzing how different merger types and microphysics treatments influence mass ejection, composition, and observable electromagnetic signals, with implications for nucleosynthesis.

Contribution

It provides the first comprehensive study of dynamical mass ejection from eccentric neutron star mergers with detailed microphysics and nucleosynthesis predictions.

Findings

Eccentric mergers eject more material than quasi-circular ones.

Ejecta are mostly broad-angle, tidally- and shock-driven, with composition affected by neutrino processes.

Nucleosynthetic yields are robust for second and third r-process peaks, but first peak yields vary and cannot fully explain Solar abundances.

Abstract

We present fully general-relativistic simulations of binary neutron star mergers with a temperature and composition dependent nuclear equation of state. We study the dynamical mass ejection from both quasi-circular and dynamical-capture eccentric mergers. We systematically vary the level of our treatment of the microphysics to isolate the effects of neutrino cooling and heating and we compute the nucleosynthetic yields of the ejecta. We find that eccentric binaries can eject significantly more material than quasi-circular binaries and generate bright infrared and radio emission. In all our simulations the outflow is composed of a combination of tidally- and shock-driven ejecta, mostly distributed over a broad $\sim 60^\circ$ angle from the orbital plane, and, to a lesser extent, by thermally driven winds at high latitudes. Ejecta from eccentric mergers are typically more neutron rich than those of quasi-circular mergers. We find neutrino cooling and heating to affect, quantitatively and qualitatively, composition, morphology, and total mass of the outflows. This is also reflected in the infrared and radio signatures of the binary. The final nucleosynthetic yields of the ejecta are robust and insensitive to input physics or merger type in the regions of the second and third r-process peaks. The yields for elements on the first peak vary between our simulations, but none of our models is able to explain the Solar abundances of first-peak elements without invoking additional first-peak contributions from either neutrino and viscously-driven winds operating on longer timescales after the mergers, or from core-collapse supernovae.