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

The formation histories of compact binary coalescences, 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 mergers. We use the state-of-the-art stellar and binary evolution code MESA and implement an up-to-date and detailed method for common envelope and mass transfer. We preform simulations for donor masses between 8-20 solar masses with a neutron star companion of 1.4 and 2.0 solar masses, at two metallicities, using varying common envelope efficiencies, and two prescriptions to determine which donors will explode in an electron-capture supernova. For a neutron star companion of 1.4 solar masses, all binary neutron star mergers are formed following a common envelope phase, while for a neutron star mass of 2.0 solar masses we identify a small subset of mergers following only stable mass transfer if the neutron star receives a large natal kick. Regardless of neutron star companion mass, we find that large supernova natal kicks are favored in the formation of binary neutron star mergers, and find more mergers at subsolar metallicity compared to solar. We observe different trends with assumed binary and stellar physics parameters compared to previous studies, and therefore conclude that detailed simulations are required in order to capture the full behavior of these systems.