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

TL;DR

This study investigates BHNS and BNS mergers originating from Population III and Population II binaries using detailed binary population synthesis with SEVN, exploring how IMF, binary initial conditions, and high-redshift star-formation histories shape merger rate densities, primary masses, and delay times. It demonstrates that Pop III mergers, while efficient, contribute less to the observed MRD than Pop II due to suppressed Pop III star formation, with BHNSs peaking at $z\sim13$ and BNSs at $z\sim15$; Pop III BHs in BHNS mergers can reach $m_1\sim50\,M_{\odot}$ and tend to have low mass ratios, offering a potential path to detect metal-free merger remnants with future detectors. The work also shows how different Pop III initial-condition assumptions affect mass distributions and uses GW event Bayes factors to assess the plausibility of Pop III versus Pop II origins for specific LVK events, identifying GW191219 as more compatible with Pop III. Overall, the results provide insight into the high-redshift origins of compact binaries and their prospects for detection with current and next-generation interferometers, highlighting the role of metallicity, SFR histories, and binary evolution physics in shaping the observable GW population.

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.

Figures (13)

• Figure 1: Evolution of stellar radii for $8 \le m_{\rm ZAMS}\leq300\,{\rm M}_\odot$. We represent Pop. III stars with a solid black line and Pop. II stars with a dashed red line. The orange stars (circles) represent the beginning (end) of core-He burning. The blue circles indicate instead the pre-SN phase.
• Figure 2: Star formation rate density of Pop. III (dash-dotted magenta line, hartwig2022) and Pop. II (dashed purple line, liu2025) stars. The shaded purple region denotes the uncertainty range associated with Pop. II star formation history. As a comparison, the solid black line shows the star formation rate density of Pop. II-I stars by madau2017.
• Figure 3: Merger rate density of BHNSs as a function of the redshift, $z$. The colored lines show the distributions for all the simulated Pop. III models. The thick solid blue line represents our fiducial model for Pop. III stars (log1). The thick solid gray line is the fiducial model for Pop. II stars (kro1); the shaded gray area encompasses the uncertainties on galactic outflows and IMF parameters (see Sec. \ref{['sec:mrd_methods']}). Finally, the thick solid black line represents the MRD of Pop. II-I BHNSs estimated by sevn2023.
• Figure 4: Distribution of delay times of BHNS mergers for the fiducial models of Pop. III (continuous blue line) and Pop. II stars (dotted blue line). The dashed gray line shows the $\propto t^{-1}$ trend from dominik2012.
• Figure 5: Distribution of primary mass of BHNS mergers for the fiducial models of Pop. III (continuous blue line) and Pop. II (dotted blue line).