Black hole–neutron star and binary neutron star mergers from Population III and II stars

¹ Gran Sasso Science Institute (GSSI), Viale Francesco Crispi 7, 67100 L’Aquila, Italy
² INFN, Laboratori Nazionali del Gran Sasso, 67100 Assergi, Italy
³ Universität Heidelberg, Zentrum für Astronomie (ZAH), Institut für Theoretische Astrophysik, Albert Ueberle Str. 2, 69120 Heidelberg, Germany
⁴ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Heidelberg, Germany
⁵ Physics and Astronomy Department Galileo Galilei, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁶ INFN – Padova, Via Marzolo 8, 35131 Padova, Italy
⁷ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ INAF Osservatorio Astronomico d’Abruzzo, Via Maggini, 64100 Teramo, Italy
⁹ Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
¹⁰ LIGO Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹¹ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹³ Elizabeth S. and Richard M. Cashin Fellow at the Radcliffe Institute for Advanced Studies at Harvard University, 10 Garden Street, Cambridge, MA 02138, USA

^★ Corresponding authors: benedetta.mestichelli@gssi.it; mapelli@uni-heidelberg.de

Received: 14 June 2025
Accepted: 6 October 2025

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 ~ 15, attaining a value R_BNS(z ~ 15) ~ 15 Gpc⁻³ yr⁻¹, while the MRD of Pop. III BHNSs is maximum at z ~ 13, reaching a value of R_BHNS(z ~ 13) ~ 2Gpc⁻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 ∼20 M_⊙ and extending up to ∼50 M_⊙. Black holes in Pop. II BHNS mergers instead show a peak at ≲ 15 M_⊙. 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.