Orbital evolution of LIGO/Virgo binaries in stellar clusters driven by cluster tides, stellar encounters, and general relativity

Abstract

ABSTRACT Origin of LIGO/Virgo gravitational wave events may involve production of binaries with relativistic components in dense stellar systems – globular or nuclear star clusters – and their subsequent evolution towards merger. Orbital parameters of these binaries (the inner orbit) and their motion inside the cluster (the outer orbit) evolve due to both external agents – random encounters with cluster stars and cluster tides due to the smooth cluster potential – and the internal ones – various sources of dissipation and precession within the binary. We present a numerical framework – binary evolution in stellar clusters (besc) – that follows the evolution of the binary inner and outer orbits accounting for all these effects simultaneously, enabling efficient Monte Carlo studies. The secular effect of cluster tides is computed in the singly averaged approximation, without averaging over the outer binary orbit. As to stellar encounters, we include the effects of both close and distant flybys on the inner and outer orbits of the binary, respectively. Also, given our focus on the LIGO/Virgo sources, we include the general relativistic precession (which suppresses cluster tides at high eccentricities) and the gravitational wave emission (shrinking the binary orbit). We use besc to illustrate a number of characteristic binary evolutionary outcomes and discuss relative contributions of different physical processes. In particular, we find that stellar encounters often dominate over the cluster tides in the evolution of compact binaries. besc can also be used to study other objects in clusters, e.g. blue stragglers, hot Jupiters, X-ray binaries, etc.