A Puzzle in the Sky
Supermassive black holes at the centers of galaxies are some of the most extreme environments in the universe. When a star wanders too close, tidal forces—the difference in gravitational pull across the star's body—can tear it apart completely in a standard tidal disruption event (TDE). But some stars survive, passing close enough to lose only a fraction of their outer layers each time. These events, known as repeating partial tidal disruption events (rpTDEs), produce bursts of light as the stripped gas falls back and accretes onto the black hole.
Wide-field sky surveys have identified roughly ten rpTDEs. In four of them, astronomers noticed an intriguing pattern: the flares grew consistently fainter with each successive encounter. This dimming puzzled researchers, as previous hydrodynamical simulations predicted that flare brightness should remain roughly constant, even as the star lost progressively less mass.
The Role of Stellar Spin
A study led by astrophysicists at Syracuse University, published in The Astrophysical Journal, offers an explanation. The research, led by doctoral student Ananya Bandopadhyay, with postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin, along with collaborators at other institutions, introduces a missing variable: the star's initial spin rate.
Earlier simulations showed that a black hole exerts a gravitational torque on a star, spinning it up with each close pass. This added rotation accelerates the fallback of stripped debris, counteracting the reduction in mass loss and keeping the flares bright. But if a star is already rotating rapidly before its first encounter, tidal forces cannot spin it up significantly further. Without ongoing spin-up, the fallback timescale stays constant, so as the star loses less mass, the flares naturally fade.
Bandopadhyay described the frustration behind the work: "We were puzzled by this for two years." The team's insight came from considering a star's initial conditions, a factor previous models had overlooked.
The Hills Mechanism Connection
The study also proposes an origin for these fast-spinning stars. The Hills mechanism describes how a supermassive black hole can break apart a tightly bound binary star system, ejecting one star and capturing the other into a close orbit. Stars in such binary pairs become tidally locked, meaning they rotate exceptionally fast. A star captured via this mechanism retains that high spin while entering a short-period orbit around the black hole.
This scenario may also explain another mystery: the presence of fast-orbiting stars near Sagittarius A*, the supermassive black hole at the center of the Milky Way. The same capture process could account for some of these stars.
Coughlin noted the difficulty of forming such systems: "It is also extremely difficult to bind a star to a supermassive black hole so tightly that it orbits the black hole in a matter of months, and yet they seem to do so in rpTDEs." He added, "Ananya's work demonstrates that each of these peculiarities can be explained by the same underlying phenomenon: the tidal destruction of a binary system and the capture of one of the stars."
A Deeper Understanding
Bandopadhyay used an analogy to illustrate the difference between low- and high-mass stars: a low-mass star is like a fluffy meringue, while a higher-mass star is more onion-like, with dense, centrally concentrated layers. This structural difference affects how mass is lost over repeated encounters.
The findings not only resolve a long-standing puzzle but also offer a unified explanation for several observed phenomena, from fading flares to unusual stellar orbits. As wide-field surveys continue to scan the sky, these insights will help astronomers interpret the behavior of stars that teeter on the edge of destruction.