In 2020, a team headed by European Southern Observatory (ESO) scientists revealed the nearest black hole to Earth, which was found in the HR 6819 system, just 1000 light-years away. However, the study’s findings were challenged by other researchers, including an international team located at KU Leuven in Belgium. In a publication released today, these two teams declare that there is no black hole in HR 6819, but rather a “vampire” two-star system in an unique and brief period of life.
The initial research on HR 6819 drew a lot of interest from the press and scientists. The astronomical community’s reaction to their finding of the black hole did not surprise Thomas Rivinius, a Chile-based ESO scientist and main author on that publication. “Not only is it reasonable, but it should be that findings be scrutinized,” he argues, “especially when the results make headlines.”
Rivinius and his colleagues were confident that the best explanation for the data collected with the MPG/ESO 2.2-metre telescope was that HR 6819 was a triple system, with one star circling a black hole every 40 days and a second star in a considerably larger orbit. However, a research conducted by Julia Bodensteiner, then a PhD student at KU Leuven in Belgium, presented an alternative interpretation for the same data: HR 6819 may be a system with just two stars on a 40-day orbit and no black hole at all. This alternate scenario would need one of the stars being “stripped,” which means that it has previously lost a significant portion of its mass to the other star.
“We had reached the limit of the current data, therefore we had to resort to a different observational method to distinguish between the two scenarios given by the two teams,” explains Abigail Frost, a KU Leuven researcher who led the new study published today in Astronomy & Astrophysics.
To unravel the enigma, the two teams collaborated to gather additional, crisper HR 6819 data using ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI) (VLTI). “The VLTI was the only facility that could provide us with the definitive evidence we required to differentiate between the two possibilities,” explains Dietrich Baade, lead author of both the original HR 6819 research and the current Astronomy & Astrophysics publication. Because it made no sense to request the same observation again, the two teams collaborated, allowing them to combine their resources and expertise to discover the real nature of this system.
“The cases we were seeking for were rather obvious, highly diverse, and fairly recognizable with the correct instrument,” Rivinius explains. “We agreed that there were two light sources in the system, therefore the issue was whether they orbited each other closely, as in the stripped-star scenario, or were far away, as in the black hole scenario.”
The scientists employed both the GRAVITY instrument on the VLTI and the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO’s VLT to differentiate between the two possibilities.
“MUSE verified that there was no light companion in a larger orbit, but GRAVITY’s excellent spatial resolution was able to detect two brilliant sources separated by barely one-third of the distance between the Earth and the Sun,” Frost explains. “These findings provided the last piece of the jigsaw, allowing us to conclude that HR 6819 is a binary system with no black hole.”
“So far, our best guess is that we captured this binary system just after one of the stars pulled the atmosphere from its partner star. This is a typical event in tight binary systems, and it is often referred to in the news as “stellar vampirism.” “Bodensteiner, who is currently a fellow at ESO in Germany and an author on the new research, says. “While the donor star was being robbed of its material, the recipient star started to spin faster.”
“Capturing such a post-interaction period is exceedingly challenging since it is so brief,” Frost says. “This makes our results for HR 6819 extremely fascinating, as it provides an ideal candidate for studying how this vampirism influences the development of massive stars, and hence the production of their related phenomena like as gravitational waves and intense supernova explosions.”
The newly created Leuven-ESO joint team intends to carefully monitor HR 6819 using the VLTI’s GRAVITY sensor. The researchers will undertake a long-term study of the system to better understand its development, restrict its attributes, and use what they find to learn more about other binary systems.
In terms of the quest for black holes, the team is upbeat. “Because of their nature, stellar-mass black holes remain elusive,” adds Rivinius. “However, order-of-magnitude calculations imply that the Milky Way alone contains tens to hundreds of millions of black holes,” Baade says. It will only be a matter of time until astronomers detect them.
This study will be published in Astronomy & Astrophysics in the publication “HR 6819 is a binary system with no black hole: Revisiting the source with infrared interferometry and optical integral field spectroscopy.”
It was funded by the European Research Council (ERC) as part of the European Union’s Horizon 2020 research and innovation initiative (grant agreement number 772225: MULTIPLES; PI: Hugues Sana).
A. J. Frost (Institute of Astronomy, KU Leuven, Belgium [KU Leuven]), J. Bodensteiner (European Southern Observatory, Garching, Germany [ESO]), Th. Rivinius (ESO Chile), D. Baade (ESO), A. Mérand (ESO), F. Selman (ESO Chile), M. Abdul-Masih (ESO Chile), G. Banyard (KU Leuven), E. Bordier ( (KU Leuven).