The Primary Flare Following a Stellar Collision in a Galactic Nucleus

High-velocity stellar collisions near supermassive black holes may result in a complete disruption of the stars. The initial disruption can have energies on par with supernovae and power a very fast transient. In this work, we examine the primary flare that follows the initial transient, which arises when streams of gas from the disrupted stars travel around the central black hole and collide with each other on the antipodal side with respect to the original collision. We present a simple analytic estimate for the properties of the flare, which depends on the distance of the collision from the central black hole and on the center of mass velocity of the colliding stars. We also present the first-of-their-kind radiation-hydrodynamics simulations of a few examples of stellar collisions and postcollision flow of the ejected gas and calculate the expected bolometric light curves. We find that such postcollision flares are expected to be similar to flares that arise in tidal disruptions events of single stars.