Clues shed light on ‘unlucky’ star’s demise

This artist’s illustration depicts the “tidal disruption event” (TDE) called ASASSN-14li, which is the focus of the latest study. As a star approached too closely to the supermassive black hole at the system, the strong gravity tore the star apart. This artist’s impression depicts the aftermath of this destruction. After the star was ripped apart, some of its gas (red) was left orbiting around and falling into the black hole. A portion of the gas was driven away in a wind (blue). (Credit: J. Miller et al/NASA/CXC/University of Michigan; M.Weiss/NASA/CXC)

Researchers have made a thorough forensic study of a star that was torn apart when it ventured too close to a giant black hole and then had its insides tossed out into space.

The 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 known to have torn apart a star. Astronomers think these elements were created inside the star before it was ripped apart as it neared the black hole.

The results are published in The Astrophysical Journal Letters.

“We are seeing the guts of what used to be a star,” says Jon Miller, professor of astronomy at University of Michigan who led the study. “The elements left behind are clues we can follow to figure out what sort of star met its demise.”

Astronomers have found many examples of “tidal disruption events” in recent years, where the gravitational forces from a massive black hole destroy a star. This causes a flare, often seen in optical and ultraviolet light and X-rays, as the star’s debris is heated up. This one, called ASASSN-14li, stands out for several reasons.

At the time of discovery in November 2014 it was the closest tidal disruption to Earth (290 million light-years) in about a decade. Because of this proximity, ASASSN-14li has provided an extraordinary level of detail about the destroyed star. Miller’s team applied new theoretical models to make improved estimates, compared to previous work, of the amount of nitrogen and carbon around the black hole.

“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 says. “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.”

The star in ASASSN-14li is therefore one of the most massive—and perhaps the most massive—that astronomers have seen ripped apart by a black hole to date.

“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,” says coauthor 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.”

Earlier this year, another team of astronomers reported the “Scary Barbie” event where they estimated a star with about 14 times the mass of the sun was destroyed by a black hole. However, this is only a candidate for a tidal disruption, with the estimate of the star’s mass mainly being based on the brightness of the flare, not on a detailed analysis of material around the black hole as with ASASSN-14li.

Another exciting aspect of the ASASSN-14li result is what it means for future studies, the researchers say. Astronomers have seen moderately massive stars like ASASSN-14li’s in the star cluster containing the supermassive black hole in the center of our galaxy. Therefore, the ability to estimate stellar masses of tidally disrupted stars potentially gives astronomers a way to identify the presence of star clusters around supermassive black holes in more distant galaxies.

Until this study there was a strong possibility that the elements observed in X-rays might have come from gas released in previous eruptions from the supermassive black hole. The pattern of elements analyzed here, however, appears to have come from a single star.

Previous work published in 2017 by Chenwie Yang from the University of Science and Technology in Hefei, China, used ultraviolet data from NASA’s Hubble Space Telescope to show that there is enhanced nitrogen compared to carbon in ASASSN-14li, but by a smaller amount than Miller’s team found using X-ray data. Those authors were only able to claim that the star is more massive than 0.6 times the mass of the sun.

Additional authors of the study are from Carnegie Observatories and the University of California, Los Angeles; University of Michigan; the Center for Astrophysics, Harvard & Smithsonian; and the University of Maryland.

NASA’s Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

Source: Megan Watzke, Chandra X-ray Observatory, NASA for University of Michigan