Wolf–Rayet–compact object binaries as progenitors of binary compact objects

¹ Physics and Astronomy Department Galileo Galilei, University of Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
² INFN – Padova, Via Marzolo 8, 35131 Padova, Italy
³ Institut für Theoretische Astrophysik, ZAH, Universität Heidelberg, Albert-Ueberle-Straße 2, 69120 Heidelberg, Germany
⁴ INAF – Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁵ Univ Lyon, Univ Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France

^★ Corresponding authors; erika.korb@studenti.unipd.it; mapelli@uni-heidelberg.de

Received: 11 October 2024
Accepted: 4 February 2025

Abstract

Binaries with a Wolf–Rayet star and a compact object (WR–COs), either a black hole (BH) or a neutron star (NS), have been proposed as possible progenitors for the binary compact object mergers (BCOs) observed with gravitational wave (GW) detectors. In this work, we use the open source population synthesis code SEVN to investigate the role of WR–COs as BCO progenitors. We consider an initial population of 5 × 10⁶ binaries, and we evolve it across 96 combinations of metallicities, common envelope efficiencies, core-collapse supernova models, and natal kick distributions. We find that WR–COs are the progenitors of most BCOs, especially at high and intermediate metallicity. At Z = 0.02, 0.014, and 0.0014, more than ≳ 99% of all the BCOs in our simulations evolved as WR–COs. At Z = 0.00014, inefficient binary stripping lowers the fraction of BCOs with WR–CO progenitors to ≈83–95%. Despite their key role in BCO production, only ≈5–30% of WR–COs end their life as BCOs. We find that Cyg X-3, the only WR–CO candidate observed in the Milky Way, is a promising BCO progenitor, especially if it hosts a BH. In our simulations, about 70–100% of the Cyg X-3-like systems in the WR–BH configuration (BH mass ≤ 10 M_⊙) are BCO progenitors, in agreement with the literature. Future observations of WR–COs similar to Cyg X-3 may be the Rosetta stone to interpret the formation of BCOs.