Binary Stars Approaching Supermassive Black Holes: Tidal Break-up, Double Stellar Disruptions and Stellar Collision

Abstract: In galactic centers, stars and binaries can be injected into low-angular-momentum orbits, resulting in close encounters with the central supermassive black hole (SMBH). Previous works have shown that under different conditions, such close encounters can lead to the break-up of the binary, disruptions of both stars and collision between the stars. We use 3-body scattering experiments to characterize these different outcomes for a range of system parameters, such as $\beta_b$, the ratio of binary tidal radius to pericenter distance $r_p$ to the SMBH and the compactness of the binary. We focus on stellar collisions, which occur for a range of $\beta_b$'s, with a few to 10's percent probabilities (depending on the compactness of the binary). In gentle encounters ($\beta_b\lesssim 1$), stellar collisions occur after the pericenter passage, and the merger remnants are typically ejected from the SMBH at a small velocity. In deep encounters ($\beta_b\gtrsim 1$), collisions occur near the pericenter, with the impact velocity a few times the escape velocity of the star, and the merger remnants are typically bound to the SMBH. We suggest that stellar collisions induced by binary-SMBH encounters may produce exotic stars in galactic centers, trigger accretion flares onto the SMBH due to the mass loss, and result in bound merger remnants causing repeated partial TDEs.