  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 gravitational collapse of
their iron core. Here, starting from a sample of red-giant star collisions
simulated with the hydrodynamics code AREPO, we generate photometric and
spectroscopic observables using the nonlocal thermodynamic equilibrium
time-dependent radiative transfer code CMFGEN. Collisions from more extended
giants or stronger collisions (higher velocity or smaller impact parameter)
yield bolometric luminosities on the order of 1e43 erg/s at 1d, evolving on a
timescale of a week to a bright plateau at ~1e41 erg/s, before plunging
precipitously after 20-40d at the end of the optically-thick phase. This
luminosity falls primarily in the UV in the first days, thus when it is at its
maximum, and shifts to the optical thereafter. Collisions at lower velocity or
from less extended stars produce ejecta that are fainter but may remain
optically thick for up to 40d if they have a small expansion rate. These
collision debris show a similar spectral evolution as that observed or modeled
for blue-supergiant star explosions of massive stars, 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 like
ULTRASAT.
