Tidal disruption events from eccentric orbits and lessons learned from the noteworthy ASASSN-14ko

TL;DR

This paper investigates the tidal disruption of stars on eccentric orbits around supermassive black holes, explaining the periodic flares observed in ASASSN-14ko and predicting shorter, recurring flares for such events.

Contribution

It develops a theoretical framework for eccentric tidal disruption events, linking them to observed periodic flares and applying it to the ASASSN-14ko candidate.

Findings

Eccentric TDEs have similar tidal deformation to parabolic encounters above a critical eccentricity.

Flares from eccentric TDEs depend on SMBH mass and orbital period, with shorter durations for shorter periods.

Evolved stars can survive multiple passages, explaining recurrent flares like ASASSN-14ko.

Abstract

Stars grazing supermassive black holes (SMBHs) on bound orbits may survive tidal disruption, causing periodic flares. Inspired by the recent discovery of the periodic nuclear transient ASASSN-14ko, a promising candidate for a repeating tidal disruption event (TDE), we study the tidal deformation of stars approaching SMBHs on eccentric orbits. With both analytical and hydrodynamics methods, we show the overall tidal deformation of a star is similar to that in a parabolic orbit provided that the eccentricity is above a critical value. This allows one to make use of existing simulation libraries from parabolic encounters to calculate the mass fallback rate in eccentric TDEs. We find the flare structures of eccentric TDEs show a complicated dependence on both the SMBH mass and the orbital period. For stars orbiting SMBHs with relatively short periods, we predict significantly shorter-lived duration flares than those in parabolic TDEs, which can be used to predict repeating events if the mass of the SMBH can be independently measured. Using an adiabatic mass loss model, we study the flare evolution over multiple passages, and show the evolved stars can survive many more passages than main sequence stars. We apply this theoretical framework to the repeating TDE candidate ASASSN-14ko and suggest that its recurrent flares originate from a moderately massive ($M\gtrsim 1 \mathrm{M_\odot}$), extended ($\approx$ a few $\mathrm{R_\odot}$), evolved star on a grazing, bound orbit around the SMBH. Future hydrodynamics simulations of multiple tidal interactions will enable realistic models on the individual flare structure and the evolution over multiple flares.