Binary black holes in the pair instability mass gap
Abstract
Pair instability (PI) and pulsational PI prevent the formation of black holes (BHs) with mass ≳60 M_{⊙} from single star evolution. Here, we investigate the possibility that BHs with mass in the PI gap form via stellar mergers and multiple stellar mergers, facilitated by dynamical encounters in young star clusters. We analyse 10^{4} simulations, run with the direct Nbody code NBODY6++GPU coupled with the population synthesis code MOBSE. We find that up to ∼6 per cent of all simulated BHs have mass in the PI gap, depending on progenitor's metallicity. This formation channel is strongly suppressed in metalrich (Z = 0.02) star clusters because of stellar winds. BHs with mass in the PI gap are initially single BHs but can efficiently acquire companions through dynamical exchanges. We find that ∼21 per cent, 10 per cent, and 0.5 per cent of all binary BHs have at least one component in the PI mass gap at metallicity Z = 0.0002, 0.002, and 0.02, respectively. Based on the evolution of the cosmic star formation rate and metallicity, and under the assumption that all stars form in young star clusters, we predict that ∼5 per cent of all binary BH mergers detectable by advanced LIGO and Virgo at their design sensitivity have at least one component in the PI mass gap.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 September 2020
 DOI:
 10.1093/mnras/staa1997
 arXiv:
 arXiv:1911.01434
 Bibcode:
 2020MNRAS.497.1043D
 Keywords:

 black hole physics;
 gravitational waves;
 methods: numerical;
 binaries: general;
 stars: kinematics and dynamics;
 galaxies: star clusters: general;
 Astrophysics  High Energy Astrophysical Phenomena;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 7 pages, 3 figures, 2 tables, MNRAS, in press