Tidal Disruption Events from Eccentric Orbits and Lessons Learned from the Noteworthy 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 hydrodynamic 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 ≳ 1 M ⊙), extended (likely ≈10 R ⊙), evolved star on a grazing, bound orbit around the SMBH. Future hydrodynamic simulations of multiple tidal interactions will enable realistic models on the individual flare structure and the evolution over multiple flares.

The Unluckiest Star: A Spectroscopically Confirmed Repeated Partial Tidal Disruption Event AT 2022dbl

“…provide conclusive evidence for a repeated pTDE classification, which might occur in the next couple of years. We notice that recent simulation works of repeated pTDEs (e.g., Bandopadhyay et al 2024;C. Liu et al 2024) have reproduced the light-curve patterns similar as ASASSN-14ko, AT 2020vdq, and AT 2022dbl under several sets of stellar parameters (e.g., mass and age) and β.…”

Tidal Stripping of a White Dwarf by an Intermediate-mass Black Hole

“…In contrast to the singlepassage TDEs, in an eccentric tidal stripping, the star will undergo many pericenter passages. Eccentric Tidal stripping of a giant star and a solar-type star are studied in MacLeod et al (2013) and Liu et al (2023), respectively. For the tidal stripping of a WD, Cheng & Evans (2013) explored the relativistic effects in the tidal interaction.…”

Comparison of Different Tidal Disruption Event Light Curve Models with TiDE, a New Modular Open Source Code

“…If the star can survive the approach, it could exhibit periodic outbursts (MacLeod et al 2013;Nixon & Coughlin 2022). In Liu et al (2023) the authors showed that stars on highly eccentric orbits can produce multiple, short-lived TDEs compared to parabolic encounters, which may explain peculiar, repeating TDEs like ASASSN-14ko.…”

“…Many optical observations were produced by large surveys, like the Sloan Digitized Sky Survey (SDSS) (e.g., Komossa et al 2008;Wang et al 2011;van Velzen et al 2011;Wang et al 2012;Yang et al 2013) or the Zwicky Transient Facility (ZTF) (van Velzen et al 2019a(van Velzen et al , 2021. For fitting the light curve of a TDE many authors used the popular open source MOSFiT code (e.g., Mockler et al 2019;Nicholl et al 2020;Liu et al 2023).…”