Binary Stars Approaching Supermassive Black Holes: Hydrodynamics of Stellar Collisions, Mass Fallback, and Partial Tidal Disruption Events

Abstract When binaries are injected into low-angular-momentum orbits around a central supermassive black hole (SMBH), various outcomes can occur, including binary tidal breakup, double stellar disruptions, and stellar collision. We use hydrodynamical simulations to study stellar collisions triggered by binary–SMBH encounters, examining both head-on and grazing collisions in deep ( β b = 5) and gentle ( β b = 0.6) encounters, where β b is the ratio of the binary tidal disruption radius to the binary pericenter distance to the SMBH. Head-on collisions consistently result in appreciable mass loss (∼5%) and a single merger remnant. Grazing collisions have varied outcomes. In gentle encounters, multiple collisions typically form a single remnant with minimal mass loss (≲1%). For deep encounters, the result depends on the specific collision parameters and stellar structure: γ = 5/3 polytropic stars in our simulation produced two disturbed remnants, while solar-type stars (modeled with MESA ) in our deep-grazing run formed a single merger remnant in a low-velocity collision. All merger remnants feature extended envelopes, making them susceptible to partial tidal disruptions when they return to the SMBH. The morphology and orbital energy distribution of collision-induced debris differ significantly from those of tidal disruption event (TDE) debris of single stars. Approximately half of the collision-generated debris falls back onto the SMBH, exhibiting a distinct time evolution of the fallback rate. We suggest that such mass loss and fallback can generate electromagnetic flares that mimic weak TDEs.