|Image credit: ESO|
According to a new study, the closest black hole system (HR 6819) to Earth, about 1,000 light-years away, has no black holes. The system was first discovered in 2020. Back then, two theories were proposed, one with the presence of a black hole and one with its absence.
However, a recent study by researchers at KU Leuven University, Belgium, took the initiative to find which of the two theories is correct. They used instruments such as the European Southern Observatory Very Large Telescope (ESO, VLT) and the VLT interferometer to get decisive data to differentiate between two theories.
All the evidence reported the presence of a ‘vampire star system’ in which two stars orbit very closely and one star out of the two feasts on another. There was no sign of a distant orbiting star that would be needed for the black hole theory. “Not only is it normal, but it should be that results are scrutinised,” said Thomas Rivinius, a Chile-based ESO astronomer and lead author on the original black hole paper.
Rivinus and his team were satisfied with the explanation that they had about the data. They knew that HR6819 was a triple system having two stars and a black hole, with one star orbiting a black hole every 40 days and the other star orbiting in a wider orbit. However, Julia Bodensteiner, a PhD student at KU Leuven, proposed a different theory for the same data. She suggested that HR6819 could be a system of only two stars with an orbiting period of 40 days with no black hole. This theory indicates that one of the stars might have stripped off or had lost a huge fraction of its mass to the companion star.
“We had reached the limit of the existing data, so we had to turn to a different observational strategy to decide between the two scenarios proposed by the two teams,” said Abigail Frost, KU Leuven researcher and lead author of the new study.
The researchers used the Very Large Telescope (VLT), and Interferometer mounted on VLT (VLTI) to collect detailed information.
“The VLTI was the only facility that would give us the decisive data we needed to distinguish between the two explanations,” said Dietrich Baade, author of both the original HR 6819 study and the new paper. To reach the correct observations and avoid confusion, both the teams decided to join hands.
“The scenarios we were looking for were rather clear, very different and easily distinguishable with the right instrument,” said Rivinius. “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.”
They used astronomical instruments like MUSE (Multi-Unit Spectroscopic Explorer) and VLT Gravity to study the star system and calculate the distance between the stars.
“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,” said 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.”
“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,” explained Bodensteiner, now a fellow at ESO in Germany and an author on the new study. “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,” added Frost. “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.”
The ESO-Leuven team plans to work more closely on HR6819 and binary star systems.
Further, the findings have been published in the journal Astronomy and Astrophysics.