Black Hole - Neutron Star and Binary Neutron Star Mergers from Population III and II stars

TL;DR

This study models the formation and merger rates of binary neutron stars and black hole-neutron star systems from Population III stars, comparing them to Population II stars, and discusses implications for gravitational-wave observations.

Contribution

It provides the first detailed comparison of Pop. III and Pop. II merger properties, including rates, masses, and redshift distributions, highlighting the potential Pop. III origin of some gravitational-wave events.

Findings

Pop. III BNS and BHNS merger rates are lower than Pop. II due to star formation rates.

Pop. III BNS mergers peak at redshift ~15 with a rate of ~15 Gpc^{-3} yr^{-1}.

Pop. III BHNS mergers peak at redshift ~13 with a rate of ~2 Gpc^{-3} yr^{-1}.

Abstract

Population III (Pop.$~$III) stars are expected to be massive and to undergo minimal mass loss due to their lack of metals, making them ideal progenitors of black holes and neutron stars. Here, we investigate the formation and properties of binary neutron star (BNS) and black hole-neutron star (BHNS) mergers originating from Pop.$~$III stars, and compare them to their metal-enriched Population II (Pop.$~$II) counterparts, focusing on their merger rate densities (MRDs), primary masses and delay times. We find that, despite the high merger efficiency of Pop.$~$III BNSs and BHNSs, their low star formation rate results in a MRD at least one order of magnitude lower than that of Pop.$~$II stars. The MRD of Pop.$~$III BNSs peaks at redshift $z\sim15$, attaining a value $\mathcal{R}_{\rm BNS}(z\sim15) \sim 15\,\rm Gpc^{-3}\,yr^{-1}$, while the MRD of Pop.$~$III BHNSs is maximum at $z\sim13$, reaching a value $\mathcal{R}_{\rm BHNS}(z\sim13) \sim 2\,\rm Gpc^{-3}\,yr^{-1}$. Finally, we observe that the black hole masses of Pop.$~$III BHNS mergers have a nearly flat distribution with a peak at $\sim 20\,\rm M_{\odot}$ and extending up to $\sim 50\,\rm M_{\odot}$. Black holes in Pop.$~$II BHNS mergers show instead a peak at $\lesssim 15\,\rm M_{\odot}$. We consider these predictions in light of recent gravitational-wave observations in the local Universe, finding that a Pop.$~$III origin is preferred relative to Pop.$~$II for some events.