On the formation history of Galactic double neutron stars

TL;DR

This paper uses population synthesis to study the formation and properties of Galactic double neutron stars, comparing models with observations to understand their formation channels and physical processes.

Contribution

It introduces a comprehensive population synthesis approach that varies key physical parameters to better match observed DNS properties and formation rates.

Findings

Mass transfer from a stripped secondary is likely stable.

A mixed low and high natal kick distribution fits observations better.

Observed DNS mass distribution constrains model assumptions.

Abstract

Double neutron stars (DNSs) have been observed as Galactic radio pulsars, and the recent discovery of gravitational waves from the DNS merger GW170817 adds to the known DNS population. We perform rapid population synthesis of massive binary stars and discuss model predictions, including formation rates, mass distributions, and delay time distributions. We vary assumptions and parameters of physical processes such as mass transfer stability criteria, supernova kick distributions, remnant mass distributions and common-envelope energetics. We compute the likelihood of observing the orbital period-eccentricity distribution of the Galactic DNS population under each of our population synthesis models, allowing us to quantitatively compare the models. We find that mass transfer from a stripped post-helium-burning secondary (case BB) onto a neutron star is most likely dynamically stable. We also find that a natal kick distribution composed of both low (Maxwellian $\sigma=30\rm~km~s^{-1}$) and high ($\sigma=265\rm~km~s^{-1}$) components is preferred over a single high-kick component. We find that the observed DNS mass distribution can place strong constraints on model assumptions.