Repeating Nuclear Flares from Tidal Disruption Events — Constraints on Stellar Properties
Some electromagnetic outbursts that have been observed from the nuclei of distant galaxies exhibit flares that re-brighten on timescales of months to years. These sources, which can be explained as the repeated tidal stripping of a star by a supermassive black hole, are known as repeating partial tidal disruption events (rpTDEs), and offer a unique window into the dynamics of stars near galactic nuclei. In this talk, I will discuss recent theoretical progress made towards modeling these sources. Using a combination of semi-analytic estimates based on an adiabatic mass-loss model, and hydrodynamical simulations of the repeated tidal interactions, we show that the lightcurves of rpTDEs are strongly coupled to the stellar type and rotation rate. High-mass ($M_\star \gtrsim 1 M_\odot$) and evolved stars lose declining amounts of mass per encounter, resulting in accretion flares that are approximately evenly spaced in time with nearly the same amplitude, quantitatively reproducing the transient ASASSN-14ko. In contrast, lower-mass and less-evolved stars lose progressively more mass and produce brighter accretion flares on subsequent encounters for the same pericenter distances, leading to the rapid destruction of the star and cessation of flares. Such systems are promising candidates for recreating events such as AT2020vdq and AT2023uqm. Generating progressively declining peak amplitudes requires high-mass stars that are rapidly rotating, at several tens of percent of their breakup angular velocity. Since rapid rotation is a natural outcome of the dynamical capture of a star from a tidally locked binary, this provides strong indirect evidence for the operation of the Hills mechanism in the formation of these systems.
