NSF-BSF: Stellar Collisions in Extreme Environments

Black holes were once a controversial prediction of Einstein’s theory of gravity: general relativity. Physicists used to debate whether a black hole - a singularity of infinite density, hidden from outside observers by an event horizon - could actually exist in the Universe, or if instead they were mathematical artifacts of a (classically) incomplete theory. We now know that black holes are ubiquitous in the Universe. The center of the Milky Way contains a superrnassive black hole (SMBH) with four million times the mass of the Sun, and the centers of most galaxies contain similar objects. SMBHS are inherently interesting as extreme natural laboratories, and by studying matter near their event horizons, astrophysicists can test Einstein’s theory of general relativity in conditions we could never reproduce in terrestrial laboratories. SMBHS are also interesting for their astrophysical importance, as they inhabit and influence the most extreme stellar environments known: galactic nuclei. Galactic nuclei are the densest star systems in the Universe, where millions of stars can be crammed into a region just a few light years in radius (for comparison, the nearest star to our own Sun is more than four light years distantl). In such a crowded environment, stars will frequently have close encounters with each other, and sometimes even direct physical collisions. The outcome of these collisions is poorly understood, and the aim of this proposal is to come to a complete theoretical understanding of what happens when two stars destroy each other by colliding near a SMBH. The three key questions we will answer are as follows:

(i) If the stars collide in vacuum, how does their gaseous debris fall into the SMBH. and how much X-ray radiation does it produce as it accretes?

(ii) If the stars collide in a pre-existing gas disk (of the kind that exists around many SMBHS), how does the brightness of that disk change?

(iii) How often do these collisions happen in galactic nuclei in the first place?