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

Abstract

The detections of gravitational waves produced in mergers of binary black holes (BH) and neutron stars (NS) 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 towards 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 black hole) until gravitational wave 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 pc. For NS-NS binaries we find merger rates in the range 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 per cent. Therefore, cluster tide-driven mergers constitute a new channel that can be further explored with current and future gravitational wave detectors.

Compact-object Mergers in the Galactic Center: Evolution in Triaxial Clusters

“…In the case of a spherical star cluster without an SMBH, merger fractions are consistently low at <1% and do not vary significantly within error as a function of a out . This is consistent with the results of Hamilton & Rafikov (2019a). The addition of an SMBH to this spherically symmetric system causes an order-of-magnitude increase in the merger fractions, reaching approximately 10% as one approaches 0.1 pc.…”

“…The dynamical evolution of binaries in these environments is a complex and multiscale process, ranging from impulsive-like close encounters with other stars to long-range tidal torques arising from the SMBH and the cluster. Despite this complexity, there has been significant recent progress in this field, mainly driven by the exciting possibility that a significant fraction of compact-object mergers detected by the LIGO-Virgo collaboration may arise dynamically in these extreme environments (e.g., Antonini & Perets 2012;VanLandingham et al 2016; Bartos et al 2017;Petrovich & Antonini 2017;Stone et al 2017;Hamers et al 2018;Hoang et al 2018;Leigh et al 2018;Randall & Xianyu 2018;Hamilton & Rafikov 2019a;Zhang et al 2019).…”

“…However, the authors treated the cluster potential as a small perturbation to a three-body system and did not explore the full extent of the cluster, but rather limited themselves to the central ∼0.5 pc of a single ad hoc potential. Second, in a series of papers, Hamilton & Rafikov (2019a, 2019b, 2019c explored the secular dynamics for the full extent of the cluster and overcame the technical limitations of Petrovich & Antonini (2017) by fully accounting for the cluster tidal field in the binary's evolution. However, the authors restricted themselves to non-triaxial clusters without an SMBH.…”

Evolution of massive stellar triples and implications for compact object binary formation

“…Modelling massive stellar triples will also help to understand the astrophysical origin of the binary mergers detected by LIGO and Virgo (Abbott et al 2019(Abbott et al , 2021a The LIGO Scientific Collaboration et al 2021). It is unknown whether they resulted from isolated stellar evolution in which the binary stars harden during a common-envelope phase (Dominik et al 2012;Belczynski et al 2016a;Olejak et al 2021) or via threebody interaction with a bound hierarchical companion (Silsbee & Tremaine 2017;Antonini et al 2017Antonini et al , 2018Liu & Lai 2018;Fragione & Kocsis 2020;Martinez et al 2021), or they were driven by some dynamical interaction within a dense stellar environment, e.g., the dense cores of globular clusters (Rodriguez et al 2016;Park et al 2017;Rodriguez & Loeb 2018;Antonini & Gieles 2020a), massive young clusters (Banerjee et al 2010;Ziosi et al 2014;Di Carlo et al 2019;Fragione & Banerjee 2021), and galactic nuclei (Antonini & Rasio 2016;Petrovich & Antonini 2017;Hamilton & Rafikov 2019;Bub & Petrovich 2020;Wang et al 2021), or if the merger population formed in a combination of these channels (Zevin et al 2021).…”

The Wave-Front Equation of Gravitational Signals in Classical General Relativity

“…This is true especially in view of the experimental evidence of gravitational waves reached after intensive and long-time efforts by experimental astrophysics [6]. The matter concerns in particular highly non-linear phenomena of GR that could be explored by studies of gravitational waves produced in compact binaries mergers [7][8][9][10].…”