ANN ARBOR, Mich. – What happens when a star gets too close to a giant black hole? In a scenario studied by a University of Michigan astronomer, a star that got too close to a black hole got torn apart and its insides were tossed out into space.
A research team used NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton to study the amount of nitrogen and carbon near a black hole that was known to have torn a star apart. Astronomers believe the nitrogen and carbon were created inside the star as it neared the black hole before it was ripped apart.
“We are seeing the guts of what used to be a star,” said Jon Miller, U-M professor of astronomy who led the study. “The elements left behind are clues we can follow to figure out what sort of star met its demise.”
Tidal disruption events
When the gravitational forces of a massive black hole destroy a star and material is blown away from the black hole is known as a “tidal disruption.”
There have been many examples of tidal disruption events in recent years. The events cause a flare, which can be seen in optical and ultraviolet light, and x-rays, as the star’s debris is heated up.
When ASASSN-14li was discovered in November 2014, it was the closest tidal disruption to Earth -- only 290 million light-years away. Because of that, researchers have been able to discover a lot about the destroyed star.
“Here, we have used X-rays to look at the elemental composition of a tidal disruption event, and found a strange pattern that is consistent with a moderately massive (3 solar masses) star,” Miller said. “This gives us confidence that it really was a single star that was shredded, but it also points to an unexpectedly high stellar mass for such an event. This may tell us about the population of stars that are closest to massive black holes in other galaxies.”
‘Observing the destruction of a massive star by a supermassive black hole is spellbinding’
The star in ASASSN-14li might be the most massive one that astronomers have seen ripped apart by a black hole.
“ASASSN-14li is exciting because one of the hardest things with tidal disruptions is being able to measure the mass of the unlucky star, as we have done here,” said co-author Enrico Ramirez-Ruiz of the University of California, Santa Cruz. “Observing the destruction of a massive star by a supermassive black hole is spellbinding because more massive stars are expected to be significantly less common than lower mass stars.”
A different team of astronomers reported a “Scary Barbie” event where they estimated a star with about 14 times the mass of the sun was destroyed by a black hole. In that case, the estimate of the star’s mass is based on the brightness of the flare and not on a detailed analysis of material around the black hole.
Because researchers can estimate stellar masses of tidally disrupted stars, this means there might be a way to identify the presence of star clusters around supermassive black holes in galaxies that are further away.
Until this study, there was a possibility that what was seen in X-rays might have come from gas released during eruptions from the supermassive black hole. Researchers believe the elements analyzed in this case appear to have come from a single star.
Miller said the work done at the University of Michigan is done using orbiting telescopes that span a huge range of light.
“We are fortunate to have a very active and energetic group of scientists here that study black holes in X-rays,” Miller said.
The results of their research have been published in The Astrophysical Journal Letters.