Black holes keep tearing these stars apart, but they survive
Astronomers have found stars that repeatedly skim past supermassive black holes, surviving each encounter while producing a new burst of light. In some systems, those flares mysteriously fade with every return. Researchers now think the key may be stars that were already spinning extremely fast before being captured. That rapid rotation could explain both the fading flares and how the stars ended…
Supermassive black holes, weighing millions to billions of times the mass of our Sun, reside at the centers of most galaxies and create some of the strongest gravitational environments known. When a star ventures too close to such a black hole, the star may not be immediately destroyed and can survive multiple close encounters, leading to a phenomenon known as repeating partial tidal disruption events (rpTDEs). These events allow astronomers to observe the same star interacting with the same black hole multiple times.
However, some rpTDE systems have puzzled astronomers. Typically, each return flare from these systems is similar, but in some cases, the flares become steadily fainter over time. Previous theoretical models have struggled to explain this behavior. A new study from astrophysicists at Syracuse University suggests that the rapid spin of the star before its first close encounter with the black hole could provide the answer.
In a standard tidal disruption event (TDE), the gravitational pull from the black hole varies so much across the star that it is completely torn apart, and the resulting stellar debris falls onto the black hole, releasing light. In partial TDEs, the star loses only part of its mass, producing less light and a weaker flare. Repeating partial TDEs occur when the surviving core of the star returns for additional close encounters, shedding more material each time.
The study led by doctoral student Ananya Bandopadhyay, postdoctoral researcher Benjamin Amend, and associate professor Eric Coughlin, along with collaborators, suggests that the rate at which the star was spinning before its first encounter with the black hole may play a crucial role in the observed fading flares. In standard models, the amount of material stripped from a star during each encounter determines the brightness of the flare.
However, previous hydrodynamical simulations predicted flares with similar peak brightness even if the star lost less material in each passage.
The researchers found that if a star is already spinning rapidly before its first encounter with the black hole, it cannot be spun up as much during later passages. This means that the time it takes for the stripped material to fall back toward the black hole remains relatively steady, even as less material is lost each time. As a result, the peak fallback rate decreases, leading to fainter flares over successive encounters, which matches what astronomers have observed.
This finding raises another question: why would a star approaching a supermassive black hole already be spinning so quickly? One possible explanation is the Hills mechanism, which suggests that two stars orbiting closely around each other may be torn apart by the black hole's gravity, with one star being captured into a tight orbit around the black hole.
If this mechanism is responsible, the star could have a very fast rotation due to the Hills mechanism, and its orbit around the black hole would be extremely tight, explaining the rapid tidal forces and the observed rpTDEs.
Written by urgent.news from ScienceDaily's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
