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

TL;DR

This paper uses 3-body scattering experiments to analyze outcomes of binary stars near supermassive black holes, revealing conditions for stellar collisions, disruptions, and potential formation of exotic stars in galactic centers.

Contribution

It provides a detailed characterization of binary-SMBH interactions, including collision probabilities and outcomes, based on system parameters, expanding understanding of stellar dynamics near black holes.

Findings

Stellar collision probabilities range from a few to tens of percent depending on binary compactness.

Gentle encounters produce ejected merger remnants with small velocities.

Deep encounters result in bound merger remnants near the SMBH.

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.