Evolutionary Origins of Binary Neutron Star Mergers: Effects of Common Envelope Efficiency and Metallicity

The formation histories of compact binary mergers, especially stellar-mass binary black hole mergers, have recently come under increased scrutiny and revision. We revisit the question of the dominant formation channel and efficiency of forming binary neutron star (BNS) mergers. We use the stellar and binary evolution code MESA and implement a detailed method for common envelope and mass transfer. We perform simulations for donor masses between 7 M circle dot and 20 M circle dot with a neutron star (NS) companion of 1.4 M circle dot and 2.0 M circle dot at two metallicities, using varying common envelope efficiencies and two different prescriptions to determine if the donor undergoes core collapse or electron capture, given their helium and carbon-oxygen cores. In contrast to the case of binary black hole mergers, for an NS companion of 1.4 M circle dot, all BNS mergers are formed following a common envelope phase. For an NS mass of 2.0 M circle dot, we identify a small subset of mergers following only stable mass transfer if the NS receives a natal kick sampled from a Maxwellian distribution with velocity dispersion sigma = 265 km s-1. Regardless of the supernova prescription, we find more BNS mergers at subsolar metallicity compared to solar.