Compact Object Binary Mergers Driven By Cluster Tides: A New Channel for LIGO/Virgo Gravitational-wave Events

Abstract

Abstract 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.