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

TL;DR

This study investigates how stellar binaries interacting with a central supermassive black hole can trigger stellar collisions, mergers, and mass fallback using three-dimensional SPH hydrodynamics. By modeling equal-mass binaries around a $M_{ m BH}=10^6 M_\odot$ SMBH on parabolic orbits and exploring polytropic ($ extgamma=5/3$) and MESA solar-type stars, the authors quantify collision outcomes and the resulting debris. They find that head-on collisions typically produce merger remnants with extended envelopes and about 5% mass loss, while grazing encounters can yield either two perturbed remnants or a single merger depending on the encounter energy and stellar structure; in all cases the remnants are puffed up, increasing their likelihood of partial tidal disruption on return. The collision debris forms a broad cloud with roughly half the mass falling back to the SMBH, but the energy distribution and fallback-rate curves differ significantly from standard TDEs, suggesting possible electromagnetic flares resembling weak TDEs and enhanced chances of repeating partial TDEs in galactic nuclei. Overall, the work highlights how collision-induced mass loss and fallback can influence SMBH fueling and produce distinctive observational signatures beyond conventional TDEs.

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 ($\sim 5\%$) and a single merger remnant. Grazing collisions have varied outcomes. In gentle encounters, multiple collisions typically form a single remnant with minimal mass loss ($\lesssim 1 \%$). 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.

Figures (16)

• Figure 1: The density distribution plots of Run 1a at different times for the $z=0$ slice (the orbital plane). The density values in the plot are in visualization code units. Note that the maximum value of the color bar does not represent the actual maximum value in the plot, but is instead chosen for better visualization of the debris stream's overall structure. In the bottom row, the small inset square shows a zoomed-in version of the region inside the dashed box from the larger plot, with the color bar adjusted for better display of the central merger remnant. The white "$\times$" marks the position of the SMBH. Here, $t = 0$ approximately corresponds to the moment of collision.
• Figure 2: The same as Figure \ref{['fig:deepheadonmovie']}, but for Run 2.
• Figure 3: The same as Figure \ref{['fig:deepheadonmovie']}, but for Run 3.
• Figure 4: The same as Figure \ref{['fig:gentleheadonmovie']}, but for Run 4.
• Figure 5: Mass loss fraction as a function of time for Runs 1-4. The lines with the "$\bullet$" markers of different colors represent results with different particle numbers ($N=5, 1, 10 \times 10^5$ per star, Runs 1a, 1b, 1c). Note that $t=0$ roughly indicates the moment of the (first)