Unstable Mass Transfer from a Main-Sequence Star to a Supermassive Black Hole and Quasi-Periodic Eruptions

Abstract

We discuss the formation and evolution of systems comprised of a low-mass ($M_\star \lesssim 4 \, \rm M_\odot$) main sequence star, orbiting a $10^5-10^7 \, \rm M_\odot$ supermassive black hole with an orbital period of order $\sim$hours, and a mild eccentricity ($e\approx0.1-0.2$), episodically shedding mass at each pericenter passage. We argue that the resulting mass transfer is likely unstable, with Roche lobe overflow initially driven by gravitational wave emission, but then being accelerated by the star's expansion in response to its mass loss, undergoing a runaway process. We show that such systems are naturally produced by two-body gravitational encounters within the inner parsec of a galaxy, followed by gravitational wave circularization and inspiral from initially highly eccentric orbits. We argue that such systems can produce recurring flares similar to the recently identified class of X-ray transients known as Quasi-Periodic Eruptions, observed at the centers of a few distant galaxies.