Dynamics of recaptures, ejections, and mergers of stellar mass binaries over multiple encounters with Sgr A*

ABSTRACT A common origin for a host of stellar phenomena in galactic centres is the tidal encounter between stellar binaries and a massive black hole (MBH), known as the ‘Hills mechanism’. Following the encounter, binaries may disrupt into an ejected star and a captured one, they may merge, or survive to either fly away or come back for one or more subsequent encounters, until they are either disrupted or fly-away. In this paper, we analyse how a binary’s fate depends on its orbital parameters, by following its evolution through up to three subsequent pericentre passages. We choose an initial population of circular binaries on parabolic orbits. We present results from our restricted three-body formalism, whose strength lies in the ability to easily explore a multidimensional parameter space and make predictions independent of the binary physical properties. We find that fates depend strongly on orbital inclination, how deep the encounter is into the MBH tidal sphere and on the binary eccentricity, developed during encounters. Generally, non-retrograde trajectories, high eccentricities or deep encounters produce disruptions preferentially. Disruption is the most common fate. A significant fraction of the surviving binaries fly away at velocities typically two orders of magnitude smaller than those of ejected stars. Including multiple encounter boosts the total disruption rate by 20 per cent or more. Finally, using an example system, we investigate the effect of finite stellar sizes and lifetimes, showing that mergers occur 31 per cent of the time, and that disruptions are still boosted by ${\sim} 10~{{\ \rm per\,cent}}$ through subsequent passages.