Black hole–neutron star mergers in globular clusters

Abstract

We model the formation of black hole–neutron star (BH–NS) binaries via dynamical interactions in globular clusters. We find that in dense, massive clusters, 16–61 per cent of the BH–NS binaries formed by interactions with existing BH binaries will undergo mergers driven by the emission of gravitational radiation. If the BHs are retained by the cluster after merging with an NS, the BHs acquire subsequent NS companions and undergo several mergers. Thus, the merger rate depends critically upon whether or not the BH is retained by the cluster after the merger. Results from numerical relativity suggest that kick imparted to a ∼7 M⊙ BH after it merges with an NS will greatly exceed the cluster's escape velocity. In this case, the models suggest that the majority of BH–NS mergers in globular clusters occur within 4 Gyr of the cluster's formation and would be unobservable by Advanced LIGO. For more massive BHs, on the other hand, the post-merger kick is suppressed and the BH is retained. Models with 35 M⊙ BHs predict Advanced LIGO detection rates in the range 0.04–0.7 yr−1. On the pessimistic end of this range, BH–NS mergers resulting from binary–single star interactions in globular clusters could account for an interesting fraction of all BH–NS mergers. On the optimistic end, this channel may dominate the rate of detectable BH– NS mergers.