Repeating partial tidal disruption events occur when a supermassive black hole tears away parts of an orbiting star without completely destroying its dense core, according to research published in The Astrophysical Journal. A Syracuse University study led by doctoral student Ananya Bandopadhyay resolves why certain light flares in these systems grow progressively dimmer with each successive close pass.
Astrophysical Simulations Reveal Stellar Spin Mechanics
Supermassive black holes sit at the centers of nearly all large galaxies, holding immense gravitational influence over surrounding matter, as reported by scienmag.com. When a star ventures close enough, gravitational tidal forces create a massive disparity in pull between the star’s near and far sides. While a standard tidal disruption event destroys the star entirely, partial events allow the stellar core to survive on an eccentric orbit, returning every few months or years to shed more of its outer envelope.
Wide-field sky surveys have detected roughly ten repeating partial tidal disruption events, or rpTDEs. In at least four of these monitored systems, astronomers noted a distinct pattern: the light flares grew consistently fainter during each successive encounter. Previous hydrodynamic simulations could not replicate this dimming effect. Earlier models showed that while higher-mass stars shed decreasing amounts of mass over time, the predicted flare brightness should have remained roughly constant because stripped debris would fall back toward the black hole much faster.
Did you know? Wide-field time-domain surveys capture enormous regions of the sky repeatedly, allowing astronomers to track how a single star changes as it is gradually dismantled over multiple years.
The Role of Initial Rotation and the Hills Mechanism
To solve the mystery, the Syracuse research team—including postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin—introduced a missing variable to the hydrodynamic models: an initial high spin rate prior to the star’s first close pass. The study demonstrates that if a star already spins rapidly before encountering the black hole, tidal forces cannot spin it up significantly further during later encounters.
Without this ongoing spin-up torque from the black hole, the timescale over which stripped gas falls back remains relatively constant. Consequently, as the star sheds smaller quantities of gas on each pass, the peak fallback rate decreases. This allows the predicted light flares to finally decline in brightness, matching real-world telescope data.
The team points to the Hills mechanism to explain how these stars achieve rapid initial rotation and tight, short-period orbits. In this process, a tightly bound binary star system approaches a supermassive black hole. The black hole’s gravity breaks the binary apart, ejecting one star at high velocity while capturing the second into a close orbit. Because stars in tight binary pairs are tidally locked—matching their rotation periods to their orbital periods—they spin exceptionally fast and retain that rapid rotation upon capture.
Frequently Asked Questions
What is a repeating partial tidal disruption event?
It is an astronomical event where a supermassive black hole strips away part of a star’s outer layers during a close pass, but leaves the stellar core intact to return for future encounters that trigger repeated light flares.
Why did previous simulations fail to show fading flares?
Earlier models did not account for the star’s initial rotation rate. Without factoring in a high starting spin, previous simulations predicted that faster debris fallback times would keep flare brightness constant despite the star losing less mass.
How do stars achieve the fast spin required for these events?
According to the research team, the Hills mechanism can capture one star from a tidally locked binary system broken apart by a black hole’s gravity, preserving its rapid rotation rate in a short-period orbit.
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