Wednesday, August 10, 2022

‘Closest black hole’ star system contains no black hole

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In 2020, news that a team of astronomers located the closest back hole to Earth — in a star system just 1,000 light-years away – received a lot of coverage in the press. But new research suggests that there is, in fact, no black hole there.

In the original 2020 report, a team led by European Southern Observatory (ESO) astronomers identified the closest black hole to Earth, in the HR 6819 star system. While the discovery garnered much attention, the results of that study were contested by other researchers, including a team based out of Katholieke Universiteit Leuven (KU Leuven), a university in Belgium.

The two teams united to release a new paper, published Wednesday in the journal Astronomy & Astrophysics, which suggests that HR 6819 has no black hole.

Thomas Rivinius, an ESO astronomer and lead author on the original 2020 paper, and his colleagues thought in 2020 that HR 6819 was a triple system, with one star orbiting a black hole every 40 days and a second star in a much wider orbit.

Black holes are notoriously hard to locate. According to NASA, “By its very nature, a black hole cannot be seen.”

The ESO astronomers used a telescope in Chile to examine HR 6819, and suggested that there was something about four or five times the mass of our sun pulling on the inner star of the system, warping its orbit. At the time, they concluded that it must be a black hole and that neither star was close enough to be pulled in, therefore they orbited it.

In response to the report, Julia Bodensteiner, then a PhD student at KU Leuven, suggested a different interpretation of the data — that HR 6819 was a system with two stars on a 40-day orbit and no black hole at all. To explain the stars’ orbits, the KU Leuven researchers said one of the stars had been “stripped,” meaning that at some point, it had lost a large fraction of its mass to the other star.

Since a more powerful telescope and telescope interferometer were needed to re-examine HR 6819, the two teams decided to collaborate.

“The scenarios we were looking for were rather clear, very different and easily distinguishable with the right instrument,” Rivinius said. “We agreed that there were two sources of light in the system, so the question was whether they orbit each other closely, as in the stripped-star scenario, or are far apart from each other, as in the black hole scenario.”

The new data determined that the two stars were separated by only one-third of the distance between the Earth and the Sun, which researchers said makes it more likely that HR 6819 is a two-star system.

“Our best interpretation so far is that we caught this binary system in a moment shortly after one of the stars had sucked the atmosphere off its companion star,” Bodensteiner, now a fellow at ESO in Germany, said. This phenomenon is often referred to colloquially as “stellar vampirism.”

But the astronomers said they are excited by the news, even if it disproves the original theory about a black hole.

“Catching such a post-interaction phase is extremely difficult as it is so short,” said Abigail Frost, a KU Leuven researcher who led the new study. “This makes our findings for HR 6819 very exciting, as it presents a perfect candidate to study how this vampirism affects the evolution of massive stars, and in turn the formation of their associated phenomena including gravitational waves and violent supernova explosions.”

And as for black holes, research suggests there are possibly hundreds of millions of black holes in the Milky Way alone, waiting to be discovered. 



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