Tracking the Dimming Mystery
Astrophysicists at Syracuse University recently identified a new variable in the observation of repeating partial Tidal Disruption Events, or rpTDEs. These phenomena occur when a star wanders into the gravitational field of a supermassive black hole without being fully consumed. During these near-misses, the black hole strips material from the star, creating a periodic flare of light. As the star orbits, it loses mass and energy, which usually causes the resulting flares to dim over time. However, researchers noted that a subset of these events exhibited unexpected dimming patterns that existing models could not explain.
The research team, led by doctoral student Ananya Bandopadhyay, worked alongside postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin to analyze why certain repeating flares remained consistently dim. Their investigation focused on ten distinct events. They found that in four of these cases, the brightness levels were significantly lower than theoretical models predicted based on mass loss alone. For two years, this discrepancy remained a persistent issue in their data. They concluded that standard hydrodynamical simulations failed to account for the internal mechanics of high-mass stars caught in these gravitational cycles.
The Role of Stellar Rotation
The team turned their attention to the rotational speed of the stars involved. A star's spin on its axis acts as a resistance to the tidal torque exerted by a black hole during each pass. When a star spins rapidly, it retains its structural integrity more effectively against tidal stripping. This discovery suggests that the initial rotation rate of a star determines the intensity of the light flares observed from Earth. If a star is already spinning at a high rate, the amount of material stripped away remains relatively constant during each encounter, leading to a stable but diminished flare output.
This raises a technical question regarding how a star arrives at such a high spin rate before its first encounter with a black hole. Eric Coughlin notes that capturing a star into a tight orbit around a supermassive black hole is a difficult process, yet the data confirms it occurs in these repeating events. The team suggests the answer lies in the Hills mechanism. This process involves a binary star system passing near a black hole, where the gravitational force splits the pair. One star is ejected into space while the other is captured into a tight orbit. Because binary stars are often tidally locked, they naturally possess high spin rates before the capture occurs.
Future Observations and Galactic Context
This model provides a way to categorize stars captured by supermassive black holes like Sagittarius A* in our own galaxy. If these stars were originally part of a binary system, their high spin rates would explain the consistent dimming observed in recent surveys. Researchers are now looking to verify this by monitoring the orbital progression of stars near the center of the Milky Way. Objects like S 301, which orbits Sagittarius A* every 8.7 years at extreme velocities, offer a natural laboratory to test these hydrodynamical theories.
What happens next involves more precise tracking of these near-field stars using instruments like the GRAVITY apparatus at the Very Large Telescope. Understanding the spin states of these stars will refine our models of how supermassive black holes influence the life cycles of nearby stellar bodies. The research suggests that the interaction between gravity and rotational momentum is a key factor in predicting the long-term visibility of galactic center events. Astronomers will monitor these candidates over the coming decade to see if their flare outputs continue to match the team's spin-based simulations.