Stellar-mass black holes in young massive and open stellar clusters – IV. Updated stellar-evolutionary and black hole spin models and comparisons with the LIGO-Virgo O1/O2 merger-event data

Abstract

ABSTRACT I present a set of long-term, direct, relativistic many-body computations of model dense stellar clusters with up-to-date stellar-evolutionary, supernova (SN), and remnant natal-kick models, including pair instability and pulsation pair instability supernova (PSN and PPSN), using an updated version of ${\rm{\small NBODY7}}$ N-body simulation program. The N-body model also includes stellar evolution-based natal spins of black holes (BHs) and treatments of binary black hole (BBH) mergers based on numerical relativity. These, for the first time in a direct N-body simulation, allow for second-generation BBH mergers. The set of 65 evolutionary models have initial masses $10^4{\!-\!}10^5\, \mathrm{M}_{\odot }$, sizes 1–3 pc, metallicity 0.0001–0.02, with the massive stars in primordial binaries and they represent young massive clusters (YMC) and moderately massive open clusters (OC). Such models produce dynamically paired BBH mergers that agree well with the observed masses, mass ratios, effective spin parameters, and final spins of the LVC O1/O2 merger events, provided BHs are born with low or no spin but spin-up after undergoing a BBH merger or matter accretion on to it. In particular, the distinctly higher mass, effective spin parameter, and final spin of GW170729 merger event is naturally reproduced, as also the mass asymmetry of the O3 event GW190412. The computed models produce intermediate-mass, $\sim 100\, \mathrm{M}_{\odot }$ BBH mergers with primary mass within the ‘PSN gap’ and also yield mergers involving remnants in the ‘mass gap’. They also suggest that YMCs and OCs produce persistent, Local-Universe GW sources detectable by LISA. Such clusters are also capable of producing eccentric LIGO-Virgo mergers.