2019ApJ...881L..13H

The detections of gravitational waves (GWs) produced in mergers of binary black holes (BHs) and neutron stars (NSs) by LIGO/Virgo have stimulated interest in the origin of the progenitor binaries. Dense stellar systems-globular and nuclear star clusters-are natural sites of compact object binary formation and evolution toward merger. Here we explore a new channel for the production of binary mergers in clusters, in which the tidal field of the cluster secularly drives the binary to high eccentricity (even in the absence of a central massive BH) until GW emission becomes important. We employ the recently developed secular theory of cluster tide-driven binary evolution to compute present day merger rates for BH-BH, NS-BH, and NS-NS binaries, varying cluster potential and central concentration of the binary population (but ignoring cluster evolution and stellar flybys for now). Unlike other mechanisms, this new dynamical channel can produce a significant number of mergers out to cluster-centric distances of several parsecs. For NS-NS binaries we find merger rates in the range of 0.01-0.07 Gpc^–3 yr^–1 from globular clusters and 0.1-0.2 Gpc^–3 yr^–1 from cusped nuclear clusters. For NS-BH and BH-BH binaries we find small merger rates from globular clusters, but a rate of 0.1-0.2 Gpc^–3 yr^–1 from cusped nuclear clusters, contributing to the observed LIGO/Virgo rate at the level of several percent. Therefore, cluster tide-driven mergers constitute a new channel that can be further explored with current and future GW detectors.