The progenitors of compact-object binaries: impact of metallicity, common envelope and natal kicks

Abstract

Six gravitational wave events have been reported by the LIGO-Virgo collaboration (LVC), five of them associated with black hole binary (BHB) mergers and one with a double neutron star (DNS) merger, while the coalescence of a black hole-neutron star (BHNS) binary is still missing. We investigate the progenitors of double compact object binaries with our population-synthesis code MOBSE. MOBSE includes advanced prescriptions for mass loss by stellar winds (depending on metallicity and on the Eddington ratio) and a formalism for core-collapse, electron-capture and (pulsational) pair instability supernovae. We investigate the impact of progenitor's metallicity, of the common-envelope parameter $\alpha{}$ and of the natal kicks on the properties of DNSs, BHNSs and BHBs. We find that neutron-star (NS) masses in DNSs span from 1.1 to 2.0 M$_\odot$, with a preference for light NSs, while NSs in merging BHNSs have mostly large masses ($1.3-2.0$ M$_\odot$). BHs in merging BHNSs are preferentially low mass ($5-15$ M$_\odot$). BH masses in merging BHBs strongly depend on the progenitor's metallicity and span from $\sim{}5$ to $\sim{}45$ M$_\odot$. The local merger rate density of both BHNSs and BHBs derived from our simulations is consistent with the values reported by the LVC in all our simulations. In contrast, the local merger rate density of DNSs matches the value inferred from the LVC only if low natal kicks are assumed. This result adds another piece to the intricate puzzle of natal kicks and DNS formation.