Dynamical Evolution of Stellar Binaries in Galactic Centers

Abstract Stellar binaries in galactic centers are relevant to several observable phenomena, including hypervelocity stars, X-ray binaries, and mergers of stars and compact objects; however, we know little about the properties of these binaries. Past works have suggested that a small fraction of them should contract to a few stellar radii or collide due to the cooperation of stellar tides and the eccentricity oscillations induced by the strong tidal field of the central massive black hole. We revisit this model with several updates. We first argue that when a binary’s pericenter separation is driven down to a few stellar radii, diffusive excitation of stellar tides should quickly contract the orbit, saving the stars from collision. Instead, the stars should end up as a very tight binary. We then show that vector resonant relaxation and perturbations from passing stars—effects not included in past models—dramatically increase the prevalence of such encounters. In numerical experiments, we find that one in five binaries around 1 pc from Sgr A* should tidally contract in this way while still on the main sequence. This rate climbs to three in five around 0.01 pc, inward of which it plateaus. We briefly discuss observable implications of these results, with particular attention to young stellar binaries in the Galactic Center.