Scientists at Syracuse University have uncovered a new explanation for a cosmic mystery: why some stars that repeatedly orbit supermassive black holes produce flares that dim over time. New computer simulations suggest a star's rapid spin before its encounter with a black hole determines how much material it loses and how brightly it shines during these dramatic events. This research offers a clearer picture of how black holes interact with stars and could help astronomers understand these powerful objects better.
When Stars Meet Black Holes
When a star ventures too close to a supermassive black hole, the intense gravitational pull can tear the star apart. This destructive process is known as a tidal disruption event, or TDE. The black hole's gravity stretches the star into a long, thin stream of material, a phenomenon often called "spaghettification."This stellar debris then wraps around the black hole, forming an accretion disk. As this material spirals inward and falls into the black hole, it heats up and releases a powerful burst of light, creating a bright flare that astronomers can observe for days or months.[livescience+10]
Not all stars are completely destroyed in these encounters. Some stars only lose a portion of their mass and survive, continuing their orbit around the black hole. These events are called repeating partial tidal disruption events, or rpTDEs. Over time, these surviving stars can return for more close encounters, leading to repeated flares.However, astronomers have been puzzled by a pattern in some of these rpTDEs: successive flares after the initial event often appear much dimmer.Existing models struggled to explain why these flares would fade.[universetoday+6]
Rapid Spin Holds the Key
Astrophysicists at Syracuse University, led by doctoral student Ananya Bandopadhyay, along with postdoctoral researcher Benjamin Amend and associate professor Eric Coughlin, propose that a star's initial spin rate is the missing piece to this puzzle. Their hydrodynamical simulations suggest that a star already spinning rapidly before its first encounter with a black hole behaves differently during subsequent close passes.[universetoday+2]
Normally, as a star gets closer to a black hole, the black hole's tidal forces not only strip away material but also exert a torque that speeds up the star's rotation with each encounter. This increased spin typically helps maintain the brightness of subsequent flares.However, the Syracuse team found that if a star is already spinning very fast, it does not experience the same large increase in spin during each interaction. When the star's spin rate does not significantly increase, less material is stripped away during each close passage, leading to a dimmer flare.This finding aligns with the observed fading pattern in some rpTDEs.[scitechdaily+6]
The question then becomes, how do stars acquire such rapid spins before encountering a black hole? The researchers point to a process called the Hills mechanism. This occurs when two closely orbiting stars, known as a binary pair, pass near a supermassive black hole. The black hole's strong gravitational pull can separate the binary, ejecting one star into space while capturing the other. If the original binary pair was very close, the stars would likely be "tidally locked," meaning they already possessed a high spin rate. The captured star would then retain this rapid spin as it begins its new orbit around the black hole, setting the stage for the observed dimming flares.[universetoday+2]
Unlocking Black Hole Secrets
Understanding these star-black hole interactions is crucial for learning more about black holes themselves. Black holes are defined by only two main properties: their mass and their spin. While astronomers have successfully measured the masses of many black holes, measuring their spin has been much more challenging.The spin of a black hole drags and twists the fabric of spacetime around it, an effect predicted by Einstein's general theory of relativity known as the Lense-Thirring effect.[skyandtelescope+2]
Astronomers are actively working to measure the spin of Sagittarius A, the supermassive black hole at the center of our own Milky Way galaxy, which has a mass about 4 million times that of our Sun.A recently discovered star, named S301, is providing a unique opportunity to do so. S301 is the fastest known star in our galaxy, reaching speeds of 25,000 kilometers per second (over 8% of the speed of light) as it orbits Sagittarius A.It comes closer to the black hole than any other observed star, approaching it at roughly 12 times the Earth-Sun distance.[eso+9]
Because S301 orbits so closely, its trajectory should be subtly affected by Sagittarius A*'s spin. By carefully tracking S301's orbit over the next decade, especially during its next closest approach in late 2031, scientists hope to directly measure the black hole's spin for the first time.Measuring black hole spin helps astronomers understand how these behemoths grow and how they might launch powerful jets of particles.The new insights into how stellar spin influences tidal disruption events add another layer to our understanding of the dynamic and violent environment around supermassive black holes.[eso+4]
