A hydrodynamical simulation of a star being ripped apart by the tidal forces of a supermassive black hole.
Credit: NASA/ S. Gezari (JHU)/ J. Guillochon (UCSC)
A study led by astrophysicists at Syracuse University resolves a long-standing puzzle surrounding repeating partial tidal disruption events, proving that a star’s initial rotation rate dictates why successive light flares grow progressively dimmer during repeated close passes around a supermassive black hole
Supermassive black holes residing at the centres of galaxies possess immense gravitational forces. When a star ventures too close to a black hole, tidal forces, the difference in gravitational pull across the star’s body, can tear it completely apart in a standard tidal disruption event (TDE). However, if the star remains slightly outside the total destruction threshold, it undergoes a partial tidal disruption.
The surviving stellar core continues on its orbit, losing a fraction of its outer envelope during each close pass every few months to several years.
As the stripped stellar gas falls back toward the black hole and accretes onto it, the material heats up and releases intense flares of light. Wide-field sky surveys have identified roughly ten repeating partial tidal disruption events (rpTDEs). In four of these systems, astronomers observed a perplexing pattern: the light flares grew consistently fainter with each successive encounter.
Previous hydrodynamic simulations failed to replicate this observed dimming. While higher-mass stars naturally shed decreasing amounts of mass over time due to their dense, centrally concentrated cores, earlier models showed that the predicted flare brightness remained roughly constant.
The simulations revealed that the black hole exerted a gravitational torque on the star, spinning it up with each pass. This added rotation accelerated the rate at which stripped debris fell back toward the black hole, counteracting the reduction in lost mass and keeping the flare peaks unnaturally bright.
Rapid initial rotation and the Hills mechanism
Published in The Astrophysical Journal, the new research led by doctoral student Ananya Bandopadhyay, postdoctoral researcher Benjamin Amend, and associate professor Eric Coughlin introduced a missing variable to the models: an initial high spin rate before the star’s first encounter with the black hole.
The researchers demonstrated that if a star is already spinning rapidly prior to its initial pass, the black hole’s tidal forces cannot spin it up significantly further during subsequent encounters. Without this ongoing spin-up, the timescale over which stripped gas falls back to the black hole stays relatively constant.
As the star sheds smaller quantities of gas on each pass, the peak fallback rate decreases, allowing the predicted light flares to finally decline in brightness as observed in real-world telescope data.
The study also provides a theoretical explanation for how these stars achieve both rapid initial rotation and the extremely tight, short-period orbits required for rpTDEs. The team points to the Hills mechanism, a process wherein a tightly bound binary star system approaches a supermassive black hole.
The black hole’s gravitational field breaks the binary apart, ejecting one star at high velocity while capturing the second star into a close orbit around itself.
Because stars in tight binary pairs become tidally locked—meaning their rotation periods match their orbital periods—they spin exceptionally fast. When the Hills mechanism captures one of these tidally locked stars, it retains its high-speed rotation rate while entering a short-period orbit around the supermassive black hole. The researchers note that this same capture mechanism may account for some of the fast-orbiting stars observed near Sagittarius A*, the supermassive black hole at the centre of the Milky Way.