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

TL;DR

This study investigates how common envelope efficiency and metallicity influence the formation of binary neutron star mergers, emphasizing the role of supernova kicks and stellar evolution parameters.

Contribution

It provides detailed simulations of binary neutron star formation, highlighting the impact of common envelope efficiency, metallicity, and supernova kicks on merger outcomes.

Findings

All mergers with 1.4 solar mass neutron stars involve a common envelope phase.

Some mergers with 2.0 solar mass neutron stars occur via stable mass transfer with large natal kicks.

More mergers are predicted at subsolar metallicity than at solar metallicity.

Abstract

The formation histories of compact binary mergers, especially stellar-mass binary-black hole mergers, have recently come under increased scrutiny and revision. In this paper we revisit the question of the dominant formation channel and efficiency of forming binary neutron-star mergers. We use the 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 for electron-capture supernovae. In contrast to the case of binary-black hole mergers, 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.