Formation of the First Two Black Hole-Neutron Star Mergers (GW200115 and GW200105) from Isolated Binary Evolution

Abstract

In this work we study the formation of the first two black hole-neutron star (BHNS) mergers detected in gravitational waves (GW200115 and GW200105) from massive stars in wide isolated binary systems - the isolated binary evolution channel. We use 560 BHNS binary population synthesis model realizations from Broekgaarden et al. (2021a) and show that the system properties (chirp mass, component masses and mass ratios) of both GW200115 and GW200105 match predictions from the isolated binary evolution channel. We also show that most model realizations can account for the local BHNS merger rate densities inferred by LIGO-Virgo. However, to simultaneously also match the inferred local merger rate densities for BHBH and NSNS systems we find we need models with moderate kick velocities ($\sigma\lesssim 10^2\,\rm{km}\,\rm{s}^{-1}$) or high common-envelope efficiencies ($\alpha_{\rm{CE}}\gtrsim 2$) within our model explorations. We conclude that the first two observed BHNS mergers can be explained from the isolated binary evolution channel for reasonable model realizations.