Collisions of red giants in galactic nuclei

Abstract

ABSTRACT In stellar-dense environments, stars can collide with each other. For collisions close to a supermassive black hole (SMBH), the collisional kinetic energy can be so large that the colliding stars can be destroyed, potentially releasing an amount of energy comparable to that of a supernova. These black hole-driven disruptive collisions have been examined mostly analytically, with the non-linear hydrodynamical effects being left largely unstudied. Using the moving-mesh hydrodynamics code arepo, we investigate high-velocity (>103 km s−1) collisions between 1 M⊙ giants with varying radii, impact parameters, and initial approaching velocities, and estimate their observables. Very strong shocks across the collision surface efficiently convert ${\gtrsim} 10~{{\ \rm per\ cent}}$ of the initial kinetic energy into radiation energy. The outcome is a gas cloud expanding supersonically, homologously, and quasi-spherically, generating a flare with a peak luminosity ≃1041–1044 erg s−1 in the extreme ultraviolet band (≃10 eV). The luminosity decreases approximately following a power law of t−0.7 initially, then t−0.4 after t ≃ 10 d at which point it would be bright in the optical band (≲1eV). Subsequent, and possibly even brighter, emission would be generated due to the accretion of the gas cloud on to the nearby SMBH, possibly lasting up to multiyear time-scales. This inevitable BH–collision product interaction can contribute to the growth of BHs at all mass scales, in particular, seed BHs at high redshifts. Furthermore, the proximity of the events to the central BH makes them a potential tool for probing the existence of dormant BHs, even very massive ones which cannot be probed by tidal disruption events.