Further Evidence for the Bimodal Distribution of Neutron Star Masses

TL;DR

This paper provides evidence for a bimodal distribution of neutron star masses, linking lower masses to electron-capture supernovae and higher masses to Fe-core collapse supernovae, with implications for binary system formation.

Contribution

It strengthens the case for a bimodal neutron star mass distribution and associates specific mass groups with distinct supernova formation channels.

Findings

Four neutron stars likely formed via electron-capture supernovae with masses around 1.25 solar masses.

Ten neutron stars are associated with Fe-core collapse supernovae with masses around 1.35 solar masses.

No observed increase in double neutron star formation probability from electron-capture supernovae.

Abstract

We use a collection of 14 well-measured neutron star masses to strengthen the case that a substantial fraction of these neutron stars was formed via electron-capture supernovae (SNe) as opposed to Fe-core collapse SNe. The e-capture SNe are characterized by lower resultant gravitational masses and smaller natal kicks, leading to lower orbital eccentricities when the e-capture SN has led to the formation of the second neutron star in a binary system. Based on the measured masses and eccentricities, we identify four neutron stars, which have a mean post-collapse gravitational mass of ~1.25 solar masses, as the product of e-capture SNe. We associate the remaining ten neutron stars, which have a mean mass of 1.35 solar masses, with Fe-core collapse SNe. If the e-capture supernova occurs during the formation of the first neutron star, then this should substantially increase the formation probability for double neutron stars, given that more systems will remain bound with the smaller kicks. However, this does not appear to be the case for any of the observed systems, and we discuss possible reasons for this.