2024A&A...682A..58D

High-velocity stellar collisions driven by a supermassive black hole (BH) or BH-driven disruptive collisions in dense, nuclear clusters can rival the energetics of supergiant star explosions following the gravitational collapse of their iron core. Starting from a sample of red-giant star collisions simulated with the hydrodynamics code AREPO, we generated photometric and spectroscopic observables using the nonlocal thermodynamic equilibrium time-dependent radiative transfer code CMFGEN. Collisions from more extended giants or more violent collisions (with higher velocities or smaller impact parameters) yield bolometric luminosities on the order of 10⁴³ erg s^–1 at 1 d, evolving on a timescale of a week to a bright plateau at ∼10⁴¹ erg s^–1 before plunging precipitously after 20-40 d at the end of the optically thick phase. This luminosity falls primarily in the UV in the first few days, thus when it is at its maximum, and shifts to the optical thereafter. Collisions at lower velocities or from less extended stars produce ejecta that are fainter but can remain optically thick for up to 40 d if they have a low expansion rate. This collision debris shows a similar spectral evolution as that observed or modeled for Type II supernovae from blue-supergiant star explosions, differing only in the more rapid transition to the nebular phase. Such BH-driven disruptive collisions should be detectable by high-cadence surveys in the UV such as ULTRASAT.