Black Holes Tear Stars Apart
Stars survive black hole encounters, but flares fade over time
Introduction To Tidal Disruption Events
At the heart of most galaxies lies a supermassive black hole, weighing millions or even billions of times more than the sun. These enormous objects create some of the strongest gravitational environments known in the universe. When a star passes too close to one of these black holes, it can be torn apart, resulting in a tidal disruption event (TDE). However, some stars survive this encounter and return for additional close passes, generating a fresh burst of light each time. These events are known as repeating partial tidal disruption events (rpTDEs) and allow astronomers to observe the same star interacting with the same black hole multiple times.
The Mystery Of Fading Flares
Wide-field time-domain surveys have made it possible to track objects whose brightness changes over time, enabling the discovery of rpTDEs. However, some of these systems have presented astronomers with a mystery. Instead of producing similar flares on each return, they become steadily fainter. Theoretical models have struggled to reproduce this behavior, leaving astronomers puzzled. Recent research from astrophysicists at Syracuse University suggests that a previously underappreciated property of the star could provide the answer: how rapidly it was spinning before its first close encounter with the black hole.
Understanding Tidal Disruption Events
In a standard TDE, the gravitational pull from a black hole varies so strongly from one side of a nearby star to the other that the star is completely torn apart. The resulting stellar debris begins falling toward the black hole, releasing light over periods ranging from days to months. Not every encounter ends with the star being completely destroyed. If a star passes close to a black hole without crossing the threshold for total disruption, it can lose only part of its mass, producing a partial TDE. During a repeating partial TDE, the star's surviving core remains in orbit and returns for additional close encounters, shedding more material each time.
The Role Of Stellar Spin
The amount of material stripped from a star during repeated encounters depends in part on the star's internal structure. A low-mass star can become increasingly susceptible to the black hole's tidal forces, while a higher mass star can lose its outer layers while its dense core remains comparatively unchanged. However, the spin of the star also plays a crucial role. As the star passes close to the black hole, the tidal forces apply torque, causing the star to rotate faster after each close encounter. This increased rotation can affect the amount of material stripped from the star, leading to fainter flares over time.
Implications And Future Outlook
The discovery of the role of stellar spin in rpTDEs provides new insights into these complex events. By understanding how the spin of the star affects the amount of material stripped during each encounter, astronomers can better model and predict the behavior of these systems. This research also highlights the importance of continued monitoring of rpTDEs, as each new observation can provide valuable information about the behavior of supermassive black holes and their interactions with nearby stars. As wide-field time-domain surveys continue to scan the sky, astronomers may uncover even more examples of rpTDEs, allowing for further study and refinement of our understanding of these extraordinary events.
Sources
This is an original synthesis by Qivorane based on reporting from the outlets below.