Hydrodynamic simulations suggest that a star that was spinning rapidly before it was captured in orbit around a supermassive black hole loses less material with each pass, so the repeated flares become weaker. The same scenario may also explain how the star arrived at such a tight orbit.
A star's rapid rotation before its first encounter with a supermassive black hole may explain why flares in some recurring systems become weaker. New simulations show that the initial rotation changes the rate at which material torn from the star returns to the black hole—and thus allows the flare to weaken with each pass.
At the center of most galaxies is a supermassive black hole, a body millions to billions of times more massive than the sun. Its gravity is strong enough to tear apart a star that gets too close, but not every encounter ends in total destruction.
Some stars survive. After a close pass tears off part of the star, the remaining core can continue to orbit the black hole and return months or years later for another encounter. Each pass removes more material and produces another burst of light. Astronomers call this phenomenon a repeating partial tidal disruption event, or rpTDE).
Time-domain wide-field sky surveys, which repeatedly scan large areas and look for changes in brightness, make it possible to track the same star and black hole over several encounters. However, some systems have presented astronomers with a persistent problem: Instead of producing similar flares each time, they gradually fade.
Of the approximately ten repeating systems identified so far, four show this decay pattern. Existing theoretical models have had difficulty reproducing it. Astrophysicists fromSyracuse University It is now suggested that the explanation lies in a property of the surviving star: its rotational speed even before the first approach to the black hole.
The study, published in The Astrophysical Journal, was led by doctoral student Ananya Bandopadhyay (Ananya Bandopadhyay), in collaboration with Benjamin Amand and Eric Coughlin of Syracuse University and researchers from other institutions.
Why some flare-ups are getting weaker
A tidal disruption event (TDE) occurs when the difference in gravitational force exerted by the black hole on the near and far sides of a star becomes strong enough to tear the star apart completely.
The resulting fragments fall back toward the black hole and are absorbed. As the material loses energy, it produces radiation that can last for days to months. The black hole itself does not emit light, so the temporary feeding of stellar fragments allows astronomers to indirectly study a body that is otherwise almost invisible.
A star passing slightly further from the black hole could escape complete annihilation, but still lose some of its mass and cause a partial TDE. If the remaining core is still bound to the black hole, it can return repeatedly and shed additional material in close passes, at intervals of a few months to a few years.
The amount of material a star sheds during each transit depends in part on its internal structure. Bandopadia likens a low-mass star to an airy pancake, which becomes increasingly vulnerable to tidal forces. A more massive star, on the other hand, is built in a sort of onion-like layering and is more concentrated at its center. It can shed its outer layers while the dense central region remains largely unaffected, and therefore shed progressively less material with each encounter.
This difference explains why different rpTDE events behave differently, but it does not solve the problem of fading flares. Ostensibly, less material being torn away from the star should result in a dimmer flare. ButHydrodynamic simulations Previous studies have shown otherwise: even when the amount of material lost is small from encounter to encounter, the simulated flares remain about the same brightness.
“We wondered about it for two years,” says Bandopadhyay.
The rotation of the star resolves the discrepancy.
In a previous study, the team discovered another effect of the black hole's gravity. In addition to tearing material away from the star, tidal forces exert a torque on it that causes it to spin faster during each close encounter. So, while less material falls back toward the black hole, it returns in a shorter time frame—and the predicted flare remains more or less as bright.
The result showed that a decrease in mass loss alone is not enough. For the flares in the simulation to weaken as astronomers actually see, a “new ingredient” was required, as Bandopadia put it: a star that was already spinning rapidly before the first encounter with the black hole.
The new study found that a star with a significant initial rotation does not receive the same large increase in rotational speed with each transit. Without this acceleration, the time it takes for the material to be torn off to return to the black hole remains relatively constant.
This leads to a different result than the previous models: if less material is torn off with each pass but the duration of its return to the black hole remains the same, the peak rate of material infall decreases — and therefore the predicted flare can weaken with each additional encounter.
In the simulations, the researchers examined main-sequence stars of one or more solar masses undergoing partial tidal disruptions near a black hole of about a million solar masses. The flares were attenuated when the star was initially rotating at high speed and in a direction consistent with its orbital motion.
Breakup of a binary system may explain the rotation
The finding raises another question: Why would a star in such a system be spinning so fast before the first encounter? Furthermore, it is very difficult to trap a star in such a tight orbit around a supermassive black hole that it completes the orbit in just months—and yet, such orbits are seen in rpTDE events.
Hills mechanism could explain both features. In this scenario, two stars orbiting each other in a tight orbit pass by a supermassive black hole. The black hole breaks up the binary system: one star is thrown away at speed, while the other is captured by its gravity.
In a tight binary system, the stars may be tidally locked, meaning that each star rotates on its axis at the same rate as the pair orbit each other. The tighter the system, the shorter the orbital period and the faster the tidally locked star rotates. For one of the stars to be trapped in the short orbit seen in rpTDE events, the original system would have to be extremely tight—and this is exactly the condition that could give the star a rapid rotation even before the first partial disruption.
"Ennia's work shows that all these oddities can be explained by the same basic phenomenon: the tidal disruption of a binary system and the capture of one of the stars," says Coughlin. "Theoretically, this is a big step forward in understanding the physics at work in these systems."
According to Coughlin, the Hills mechanism could have shaped other stellar populations as well. Some of the stars orbiting the Sagittarius A*, the supermassive black hole at the center of the Milky Way, were placed in their orbits in a similar way. The findings may therefore help understand the properties of stars in our "cosmic backyard."
Questions and Answers
What is a recurring partial tidal disruption event?
This is an event in which a star passes close to a supermassive black hole, loses only a fraction of its mass and survives. The remaining core continues its orbit and returns for further encounters, each of which may produce a new flare.
Why are flares in some systems getting weaker?
According to the new simulations, a star that was already spinning rapidly before the first encounter gets less extra spin with each pass. When less material is torn off and the time it takes to return to the black hole remains the same, the peak rate of material infall decreases and the flare weakens.
What is the Hills mechanism?
This is a scenario in which a supermassive black hole breaks up a tight binary star system: one star is thrown out and the other is captured in orbit around the black hole. Tidal locking in the original system could also explain the rapid rotation of the captured star.
What does it have to do with Sagittarius A* at the center of the Milky Way?
The researchers suggest that the Hills mechanism may also explain how some stars arrived in their close orbits around Sagittarius A*, so the study may contribute to understanding the environment of the black hole in our galaxy.
The scientific article
More on the subject on the science website
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- How black holes devour stars
- The star that reaches 8% of the speed of light while orbiting the black hole at the center of the Milky Way
- The black hole at the center of the Milky Way isn't tearing everything apart.
- A double star near the black hole at the center of the Milky Way may be feeding it small gas clouds
One response
No, as usual, there is nothing new except the fact that it is new to the new scientists!