Properties of the remnant disk and the dynamical ejecta produced in low-mass black hole-neutron star mergers

TL;DR

This study uses numerical simulations to analyze matter behavior and ejecta properties in low-mass black hole-neutron star mergers across various mass ratios and equations of state, revealing dependencies on mass ratio and introducing a new analysis method.

Contribution

It introduces a novel phase space analysis method for matter during mergers and provides detailed insights into ejecta mass dependence on mass ratio.

Findings

Remnant mass outside the horizon depends weakly on mass ratio.

Ejecta mass peaks at a mass ratio of about 3.

Ejecta mass decreases steeply at low mass ratios.

Abstract

We systematically perform numerical-relativity simulations for low-mass black hole-neutron star mergers for the models with seven mass ratios $Q=M_{\rm BH}/M_{\rm NS}$ ranging from 1.5 to 4.4, and three neutron-star equations of state, focusing on properties of matter remaining outside the black hole and ejected dynamically during the merger. We pay particular attention to the dependence on the mass ratio of the binaries. It is found that the rest mass remaining outside the apparent horizon after the merger depends only weakly on the mass ratio for the models with low mass ratios. It is also clarified that the rest mass of the ejecta has a peak at $Q \sim 3$, and decreases steeply as the mass ratio decreases for the low mass-ratio case. We present a novel analysis method for the behavior of matter during the merger, focusing on the matter distribution in the phase space of specific energy and specific angular momentum. Then we model the matter distribution during and after the merger. Using the result of the analysis, we discuss the properties of the ejecta.