Astronomers rule out a black hole

It was believed to be closest to Earth – but now the alleged black hole in HR 6819 has finally turned out to be a phantom: instead of a triple star system with a “dark third” it is a special binary star system, new investigation shows. Apparently, the partner there had recently fallen victim to “stellar vampirism”: one star was sucking mass from the other, giving the system its unusual characteristics.

Their gravitational pull is so strong that even light cannot escape them: many people are interested in mysterious black holes in the universe because of their spectacular physical characteristics. A study from two years ago, in which scientists reported traces of a black hole in our cosmic neighborhood, showed a media echo. The research team led by Thomas Rivinius of the European Southern Observatory (ESO) focused on the HR 6819 system about 1000 light-years away from us. system. According to their interpretation, it consists of a star orbiting an invisible object with a mass of about four solar masses every 40 days, and a second star with a much wider orbit. Scientists concluded that the “dark third” would have to be a stellar black hole. Thus, it became known as the closest black hole to date.

Explorers join forces with the doubters

But shortly after publication, other astronomers expressed doubts. According to them, the data in which Rivinius and his colleagues recognized the gravitational influence of the black hole could also be interpreted differently. Rivinius and his colleagues were neither surprised nor offended by these reactions: “It’s not only normal, it should also be that the results are questioned,” says Rivinius, “and the result, which makes headlines even more.” The team of astronomers from KU Leuven in Belgium was also one of the doubters: Julia Bodensteiner and her colleagues showed another possible explanation for the same data: HR 6819 could therefore be a system with only two stars in a 40-day orbit. However, this alternative scenario without a black hole would assume that one star has recently lost a large portion of its mass to the other star.

Eventually, both teams worked together to solve the mystery – allowing them to pool their resources and knowledge to discover the true nature of this system. “However, we reached the limit of the existing data, so we had to use a different observation strategy to choose between the two possible scenarios,” says Abigail Frost of KU Leuven, who led the study of the joint research team. The new results are based on the sharper HR 6819 observational data provided by the Very Large Telescope (VLT) and the Very Large Telescope Interferometer (VLTI).

There is no such thing as a “dark third side”.

“The scenarios we were looking for were quite clear, very different and easily distinguishable with the right tool,” says Rivinius. “We agreed that there are two light sources in the system. So the question was whether they orbit each other close to each other, as in the naked star scenario, or far apart, as in the black hole scenario, explains the astronomer. To distinguish the two proposals, the combined team used both the GRAVITY VLTI instrument and the Multi Unit Spectroscopic Explorer (MUSE) instrument on the VLT.

Thus the mystery was finally resolved: “MUSE confirmed that there is no bright companion further orbit, while GRAVITY’s high spatial resolution was able to resolve two bright sources that are separated only a third of the distance between the Earth and the Sun has been separated”, informs Frost. “This data turned out to be the last piece of the puzzle, which allows us to conclude that HR 6819 is a binary star system without a black hole.”

Proof of “stellar vampirism”

The researchers emphasize that the system is by no means “boring”. “Our best interpretation of the data so far is that we caught this binary star system just as one star sucked the atmosphere from its companion star. It’s a common phenomenon in close binaries, sometimes referred to as stellar vampirism, ”says Bodensteiner. “While the donor star was losing some of its material, the receiving star began to spin faster.”

Frost continues: “It is extremely difficult to capture this phase after the replacement because it is so short. This makes our results for HR 6819 very exciting. Because the system is an ideal candidate to study how vampirism influences the evolution of massive stars and thus the emergence of related phenomena such as gravitational waves and violent supernova explosions, ”says Frost. Therefore, the combined team now plans to continue to keep a close eye on HR 6819: they want to use the new observational data to better understand its features and obtain clues about the evolutionary history of this binary system.

Scientists also remain optimistic about the search for stellar black holes – despite the current deletion: stellar black holes will still be very difficult to find due to their nature, but apparently there is a lot of potential to be discovered. “Estimates suggest there are tens to hundreds of millions of black holes in the Milky Way itself,” says co-author Dietrich Baade of ESO. So it seems only a matter of time before some actually make themselves felt through their influence.

Source: ESO, Article: Astronomy and Astrophysics, doi: 10.1051 / 0004-6361 / 202143004

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