Recent discoveries of gravitational wave (GW) events, most likely originating from black hole (BH) + neutron star (NS) mergers, have revealed the existence of BH+NS binaries. The formation of BH+NS binaries and their merger rates through isolated binary evolution have been investigated extensively with population synthesis simulations. However, until now, a detailed stellar evolution modeling of the formation of this population has been missing.
A team of researchers from Long Jiang, Wen-Cong Chen, Thomas M. Tauris and others have performed the first complete 1D models of more than 30 BH+NS progenitor systems, which are calculated self-consistently until the iron core collapse with infall velocity exceeds 1000 km s^-1. The researchers applied the MESA code, starting from a post-common envelope binary with a short orbital period (<1 day) consisting of a BH and a zero-age main-sequence helium star that experiences stable mass transfer. These BH+NS systems are likely to merge and produce GW events within a Hubble time.
In the study, the researchers found that System C is a potential progenitor of a GW200115-like event, while Systems A and B are possible candidates for a GW200105-like event and may represent the final destiny of the X-ray binary SS433. This study provides a proof-of-concept of the final outcome of the formation of BH+NS binaries, filling a gap in the literature.
Compact objects, such as neutron stars and black holes, are fossils and probes for testing both stellar and binary evolutionary theory. Despite the existence of BH+NS binaries being predicted by population synthesis, none have been detected in our galaxy yet. With this new research, the formation and final outcome of these systems have been demonstrated, providing new insights into the formation of BH+NS binaries and their potential to produce GW events.
In conclusion, this study provides new insight into the formation of BH+NS binaries and the potential for these systems to merge and produce GW events. The results of this research may provide a starting point for further studies into the formation of compact objects and their potential impact on our understanding of both stellar and binary evolutionary theory.
Source: Jiang, L., Chen, W., Tauris, T.M., Muller, B., & Li, X. (2023). Simulations of the progenitors of black hole-neutron star gravitational wave sources. https://doi.org/10.48550/arXiv.2302.04812