Two years ago, astronomers reported the discovery of the closest black hole to the solar system. Well, it turns out that there wasn’t a black hole there after all. Follow-up observations have revealed that the three-body system called HR 6819 – thought to be two stars and a black hole – is actually just two stars. But those two stars are far from ordinary.
New observations have revealed that the system is made up of two stars orbiting each other every 40 days. One of the stars had its outer layer stripped by its companion and the team caught this just after it happened, thus creating the mystifying observations that first suggested the presence of an invisible black hole. The findings are reported in Astronomy & Astrophysics.
“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. This is a common phenomenon in close binary systems, sometimes referred to as “stellar vampirism” in the press,” co-author Dr Julia Bodensteiner, now a fellow at European Southern Observatory (ESO) in Germany, said in a statement. “While the donor star was stripped of some of its material, the recipient star began to spin more rapidly.”
"Catching such a post-interaction phase is extremely difficult as it is so short," lead author Dr Abigail Frost, from KU Leuven, said in a statement. "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.”
Once the original research was published, Bodensteiner led a study that put forward the proposal without the black hole. Discussing things with the original team they arrived to the conclusion that more observations were needed to work out which scenario was correct.
“Not only is it normal, but it should be that results are scrutinised,” said ESO’s Thomas Rivinius, lead author of the original research and co-author of this one. “And a result that makes the headlines even more so.”
Using the Multi-Unit Spectroscopic Explorer (MUSE) instrument on ESO’s Very Large Telescope, the astronomers were able to study the system in exquisite detail. It showed that there was not a star orbiting a black hole with a third one orbiting both further out. It was two stars orbiting quite close to each other.
“MUSE confirmed that there was no bright companion in a wider orbit, while GRAVITY’s high spatial resolution was able to resolve two bright sources separated by only one-third of the distance between the Earth and the Sun,” added Frost. “These data proved to be the final piece of the puzzle, and allowed us to conclude that HR 6819 is a binary system with no black hole.”
Small black holes, unless they are actively feeding are very difficult to discover. By they must be out there, so it’s just a matter of time before close-by ones are found.