From ZAMS to merger: Detailed binary evolution models of coalescing neutron star – black hole systems at solar metallicity

¹ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, CH-1290 Versoix, Switzerland
e-mail: Zepei.Xing@unige.ch
² Gravitational Wave Science Center (GWSC), Université de Genève, CH1211 Geneva, Switzerland
³ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
⁴ Department of Physics, University of Florida, 2001 Museum Rd, Gainesville, FL 32611, USA
⁵ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Magrans, 08193 Barcelona, Spain
⁶ Institut d’Estudis Espacials de Catalunya (IEEC), Carrer Gran Capità, 08034 Barcelona, Spain
⁷ Institutt for Fysikk, Norwegian University of Science and Technology, 7491 Trondheim, Norway
⁸ Electrical and Computer Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
⁹ Institute of Astrophysics, FORTH, N. Plastira 100, Heraklion 70013, Greece
¹⁰ IAASARS, National Observatory of Athens, Vas. Pavlou and I. Metaxa, Penteli 15236, Greece

Received: 15 September 2023
Accepted: 21 November 2023

Abstract

Neutron star – black hole (NSBH) merger events bring us new opportunities to constrain theories of stellar and binary evolution and understand the nature of compact objects. In this work, we investigated the formation of merging NSBH binaries at solar metallicity by performing a binary population synthesis study of merging NSBH binaries with the newly developed code POSYDON. The latter incorporates extensive grids of detailed single and binary evolution models, covering the entire evolution of a double compact object progenitor. We explored the evolution of NSBHs originating from different formation channels, which in some cases differ from earlier studies performed with rapid binary population synthesis codes. In this paper, we present the population properties of merging NSBH systems and their progenitors such as component masses, orbital features, and BH spins, and we detail our investigation of the model uncertainties in our treatment of common envelope (CE) evolution and the core-collapse process. We find that at solar metallicity, under the default model assumptions, most of the merging NSBHs have BH masses in the range of 3 − 11 M_⊙ and chirp masses within 1.5 − 4 M_⊙. Independently of our model variations, the BH always forms first with dimensionless spin parameter ≲0.2, which is correlated to the initial binary orbital period. Some BHs can subsequently spin up moderately (χ_BH ≲ 0.4) due to mass transfer, which we assume to be Eddington limited. Binaries that experience CE evolution rarely demonstrate large tilt angles. Conversely, approximately 40% of the binaries that undergo only stable mass transfer without CE evolution contain an anti-aligned BH. Finally, accounting for uncertainties in both the population modeling and the NS equation of state, we find that 0 − 18.6% of NSBH mergers may be accompanied by an electromagnetic counterpart.