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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.