2018MNRAS.474.2937C

We revisit double neutron star (DNS) formation in the classical binary evolution scenario in light of the recent Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo DNS detection (GW170817). The observationally estimated Galactic DNS merger rate of R_ MW_ = 21⁺²⁸_–14 Myr^–1, based on three Galactic DNS systems, fully supports our standard input physics model with R_MW = 24 Myr^–1. This estimate for the Galaxy translates in a non-trivial way (due to cosmological evolution of progenitor stars in chemically evolving Universe) into a local (z ≃ 0) DNS merger rate density of R_local = 48 Gpc^–3 yr^–1, which is not consistent with the current LIGO/Virgo DNS merger rate estimate (1540⁺³²⁰⁰_–1220 Gpc^–3 yr^–1). Within our study of the parameter space, we find solutions that allow for DNS merger rates as high as R_ local_ ~600⁺⁶⁰⁰_–300 Gpc^–3 yr^–1 which are thus consistent with the LIGO/Virgo estimate. However, our corresponding BH-BH merger rates for the models with high DNS merger rates exceed the current LIGO/Virgo estimate of local BH-BH merger rate (12-213 Gpc^–3 yr^–1). Apart from being particularly sensitive to the common envelope treatment, DNS merger rates are rather robust against variations of several of the key factors probed in our study (e.g. mass transfer, angular momentum loss, and natal kicks). This might suggest that either common envelope development/survival works differently for DNS (∼10-20 M_☉ stars) than for BH-BH (∼40-100 M_☉ stars) progenitors, or high black hole (BH) natal kicks are needed to meet observational constraints for both types of binaries. Our conclusion is based on a limited number of (21) evolutionary models and is valid within this particular DNS and BH-BH isolated binary formation scenario.