Binary Stars Approaching Supermassive Black Holes: Hydrodynamics of Stellar Collisions, Mass Fallback and Partial TDEs
arXiv:2504.14146 · doi:10.3847/1538-4357/ae032b
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 () and gentle () encounters, where 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 () and a single merger remnant. Grazing collisions have varied outcomes. In gentle encounters, multiple collisions typically form a single remnant with minimal mass loss (). For deep encounters, the result depends on the specific collision parameters and stellar structure: 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.
22 pages, 16 figures, accepted to ApJ
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