It was considered to be the closest to the earth – but now the alleged black hole in the HR 6819 system has finally turned out to be a phantom: instead of a triple system with a “dark third”, it is a special double star system, the new investigation results show. Apparently, a partner there recently became the victim of “stellar vampirism”: one of the stars sucked mass from the other, giving the system its unusual features.
Their gravitational pull is so strong that even light cannot escape them: many people are interested in the mysterious black holes of the universe because of their spectacular physical features. A study two years ago, in which scientists reported the traces of a black hole in our cosmic neighborhood, found media echo. The research team led by Thomas Rivinius from the European Southern Observatory (ESO) focused on the system HR 6819, which is about 1000 light years away from us. From the data obtained with the 2.2-metre telescope of the MPG/ESO, they concluded that it was is a special triple system. According to their interpretation, it consists of a star orbiting an invisible object of about four solar masses every 40 days and a second star in a much wider orbit. The researchers concluded that the “dark third” would have to be a stellar black hole. This is how it became known as the closest black hole to date.
“Discoverers” team up with doubters
But soon after the 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 is not only normal, it should also be the case that results are questioned,” says Rivinius, “and a result that makes the headlines, even more so.” A team of astronomers from KU Leuven in Belgium also belonged to 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 of the stars had lost a large part of its mass to the other star astronomically recently.
Eventually, the two teams worked together to solve the mystery – allowing them to pool their resources and knowledge to uncover the true nature of this system. “However, we had reached the limit of the existing data, so we had to use a different observation strategy to decide between the two possible scenarios,” says Abigail Frost from KU Leuven, who led the study of the combined research team. The new results are based on sharper observation data of HR 6819 provided by ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI).
There is no such thing as a “dark third party”.
“The scenarios we were looking for were pretty clear, very different, and easy to distinguish 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 closely, as in the stripped star scenario, or far apart, as in the black hole scenario,” explains the astronomer. To distinguish between the two proposals, the combined team used both the VLTI’s GRAVITY instrument and the Multi Unit Spectroscopic Explorer (MUSE) instrument on the VLT.
This is how the mystery was finally solved: “MUSE confirmed that there is no bright companion in a further orbit, while the high spatial resolution of GRAVITY was able to resolve two bright sources that are only separated by a third of the distance between Earth and were separated by the sun,” reports Frost. “These data proved to be the final piece of the puzzle, allowing us to conclude that HR 6819 is a binary star system without a black hole.”
Evidence 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 at a moment just after one of the stars sucked the atmosphere from its companion star. This is a common phenomenon in close binary systems, sometimes referred to as stellar vampirism,” says Bodensteiner. “While the donating star was losing some of its material, the receiving star started spinning faster.”
Frost continues: “It is extremely difficult to capture such a post-exchange phase because it is so brief. This makes our results for HR 6819 very exciting. Because the system is a perfect candidate to study how this vampirism influences the evolution of massive stars and thus the emergence of the associated phenomena such as gravitational waves and violent supernova explosions,” says Frost. The combined team therefore now plans to continue to keep a close eye on HR 6819: They want to use new observation data to better understand its characteristics and gain clues to the evolutionary history of this binary system.
The researchers 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 there is apparently a lot of discovery potential. “Estimates suggest that there are tens to hundreds of millions of black holes in the Milky Way alone,” says co-author Dietrich Baade from ESO. It therefore seems only a matter of time before some actually make themselves felt through their influence.
Source: ESO, technical article: Astronomy and Astrophysics, doi: 10.1051/0004-6361/202143004