Stellar rotation explains fading flares in repeated tidal disruptions ✦
Repeating partial tidal disruptions of a star by a supermassive black hole (rpTDE) can cause transients with rebrightenings after months or years. For some candidates, a sequential decrease in peak luminosity is observed — a trend not reproduced by theoretical models when the star survives multiple encounters. It is suggested that the trend is explained by the initially rapid rotation of the star, expected if it resulted from the breakup of a tight binary system near the black hole (Hills mechanism). The hypothesis was tested with hydrodynamic simulations: massive (≥ 1 M⊙) main-sequence stars, repeatedly partially disrupted by a 10^6 M⊙ black hole. Indeed, with prograde rotation at tens of percent of the critical speed, successive flares became dimmer. This is strong circumstantial evidence for the launch of stars in rpTDE via the Hills mechanism.
Context
Repeating tidal disruptions are a rare class of events where a star on a highly elongated orbit periodically loses some mass when passing close to a supermassive black hole. When Karl Schwarzschild found the first exact solution to Einstein's equations, no one imagined black holes could tear stars apart. Current hydrodynamic models predicted that for massive stars, the amount of stripped material and peak luminosity either increase or stay constant, contradicting observations of several candidates exhibiting fading flares. This points to an overlooked physical factor. Back in the mid-20th century, John Wheeler coined the term 'black hole', and since then, studying tidal disruptions around these objects has become crucial for understanding the growth of galactic nuclei.
Methods
The authors used the smoothed-particle hydrodynamics method (code PHANTOM) to simulate partial disruptions of main-sequence stars, whose structure was computed with the MESA code. The modeled stars consisted mainly of hydrogen and helium, with initial rotation up to 80% of the critical breakup speed. For constructing initial stellar models, theory tracing back to the work of Chandrasekhar was employed. The simulations covered 1 and 3 M⊙ stars on parabolic orbits around a black hole of mass 10^6 M⊙ with pericenter parameter β = 0.6–1.0. The accretion rate—a key quantity determining the flare brightness—and the evolution of core rotation over up to four consecutive passages were tracked.
Results
The results unequivocally show that when a star rotates prograde with angular frequency comparable to the orbital frequency at pericenter (Ω_p ≈ Ω_* × (M_BH/M_*)^{1/2} (R_*/r_p)^{3/2}), the tidal interaction hardly increases its spin. Instead, the mass loss from one encounter to the next decreases. For a 1 M⊙ star on the ZAMS (λ=0.7, β=0.6), the peak accretion rate dropped by a factor of ~1.5, and for a 3 M⊙ star on the TAMS (λ=0.8, β=1.0)—even more significantly. This decline persists over all four calculated passages, reproducing for the first time observations in the centers of galaxies of objects like eRASSt-J045650 (five flares) and AT2022dbl. In contrast, when there was no initial rotation, the spin quickly increased and peak luminosity grew, as in previous work. A third case—rotation faster than Ω_p—also leads to fading, but with the star spinning down. Thus, it is the combination of high initial spin and a sufficiently massive (≳1 M⊙) evolved star that leads to successively weakening flares.
Implications
These results provide strong indirect evidence for the Hills mechanism (disruption of a tight binary, with one star captured and the other ejected from the galaxy). Only such a scenario naturally gives the star rapid prograde rotation and the short orbital period observed in rpTDEs. Additionally, constraints are placed on the nature of the progenitor star: it must be massive, sufficiently evolved (so that its mean density increases during mass loss), and rapidly rotating. This is consistent with the overall dynamical picture in the centers of massive galaxies.
Future development
Future studies need to test retrograde rotations (expected to cause brightness increases, unless the spin exceeds Ω_p). Of particular interest is the case of AT2018fyk, where the luminosity drop reached an order of magnitude, which might indicate relativistic effects with a more massive black hole (M∼10^7.7 M⊙). With the launch of JWST and the start of LSST surveys, the discovery of many new rpTDEs is expected. Joint photometric and spectroscopic surveys (e.g., with JWST) will enable precise measurements of luminosity decline and periods to test our predictions.
Impact
This work will impact our understanding of the growth of supermassive black holes through accretion of stellar material, the statistics of hypervelocity stars, and the classification of nuclear transients, as some repeating TDEs may mimic supernovae or AGN activity.
Next steps
A systematic parameter scan (spin, stellar mass, β) and comparison with the full observed sample are needed to identify allowed regions. Including more realistic physics (radiation, magnetic fields, relativistic corrections) will improve prediction accuracy.
Key open problems
The study directly addresses the unsolved problem of short-period orbit formation in tidal disruption events and the connection between binary dynamics in the nuclei of galaxies and accretion activity. It also deepens understanding of the feedback between stellar populations and the growth of central black holes.
🎯 When a binary system disrupts via the Hills mechanism, one star is captured into a tight orbit while the other is flung away at hundreds of kilometers per second—thus hypervelocity stars are born, leaving the galaxy forever.
Key numbers
- black hole mass (model): 10^6 M⊙
- stellar spin λ: 0.7–0.8 of breakup frequency
- drop in peak luminosity: by a factor of 1.5–10
- tidal encounter parameter β: 0.6–1.0
- number of flares in eRASSt-J045650: 5