Black Hole-Neutron Star Mergers in Globular Clusters

TL;DR

This paper models the formation and merger rates of black hole-neutron star binaries in globular clusters, highlighting the impact of black hole retention on gravitational wave detection prospects.

Contribution

It introduces a detailed dynamical model for BH-NS binary formation in globular clusters, emphasizing the role of black hole retention and merger kicks.

Findings

16-61% of BH-NS binaries in dense clusters merge within 4 Gyr

Detection rates for Advanced LIGO are estimated at 0.04-0.7 per year

Most early mergers may be unobservable due to high recoil kicks

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% 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 a 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_sun BH after it merges with a 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 Gyrs 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_sun BHs predict Advanced LIGO detection rates in the range 0.04 - 0.7 per year. 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.