The nearest black hole to Earth, situated about 1,000 light-years away in the HR 6819 system, was discovered by a team headed by European Southern Observatory (ESO) researchers in 2020. However, other researchers disputed their findings, including an international team located at KU Leuven in Belgium. These two teams have joined forces in a study released today to declare that HR 6819 is not a black hole but rather a “vampire” two-star system at a unique and short-lived stage of its existence.
Both the press and scientists paid close attention to the initial research on HR 6819. The astronomical community’s reaction to their finding of the black hole did not surprise Thomas Rivinius, a Chile-based ESO scientist and the main author of that publication. “It’s not only reasonable, but it should be,” he argues, “that outcomes be analyzed, and a result that hits the headlines even more so.”
Rivinius and his colleagues were confident that the best explanation for the data collected with the MPG/ESO 2.2-meter telescope was that HR 6819 was a triple system, with one star around a black hole every 40 days a second star orbiting in a considerably larger orbit. However, a research headed by Julia Bodensteiner, a Ph.D. student at KU Leuven in Belgium at the time, 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 would imply that it had lost a significant portion of its mass to the other star at some point in the past.
“We’d hit the limit of the available data, so we had to switch to a different observational method to choose between the two scenarios suggested by the two teams,” explains Abigail Frost of KU Leuven, who led the new study published today in Astronomy & Astrophysics.
The two teams collaborated in gathering additional, finer data of HR 6819 using ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer to solve the enigma (VLTI). Dietrich Baade, the author of both the original HR 6819 research and the current Astronomy & Astrophysics publication, states, “The VLTI was the sole facility that would offer us the definitive data we required to discriminate between the two possibilities.” Because it was pointless to request the same observation again, the two teams merged, allowing them to combine their resources and expertise to discover the real nature of this system.
“We were searching for circumstances that were really obvious, highly diverse, and clearly recognizable with the correct instrument,” Rivinius explains. “We agreed that the system had two light sources, thus the issue was whether they orbited each other closely, as in the stripped-star scenario, or far away, as in the black hole scenario.”
The scientists employed the GRAVITY instrument on the VLTI and the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO’s VLT to differentiate between the two suggestions.
“MUSE verified that there was no light companion in a larger orbit,” adds Frost, “but GRAVITY’s great spatial resolution was able to detect two brilliant sources separated by barely one-third the distance between the Earth and the Sun.” “These findings completed the picture, allowing us to infer that HR 6819 is a binary system with no black hole.”
“So far, our best guess is that we saw this binary system just after one of the stars absorbed the atmosphere off its partner star. This is a regular occurrence in tight binary systems that has been dubbed “stellar vampirism” in the media “Bodensteiner, who is currently a fellow at ESO in Germany and a co-author on the new research, says. “As the material from the donor star was taken away, the recipient star started to spin faster.”
“Because it is so brief, catching such a post-interaction period is incredibly challenging,” Frost says. “This makes our results for HR 6819 extremely fascinating, as it provides a great candidate for studying how this vampirism influences the development of big stars, and hence the production of their related phenomena like gravitational waves and intense supernova explosions,” says the team.
HR 6819 will now be intensively monitored by the newly created Leuven-ESO joint team, which will use the VLTI’s GRAVITY instrument. The researchers will collaborate to investigate the system over time to understand its development better, restrict its attributes, and apply what they learn to other binary systems.
The team is enthusiastic about its quest for black holes. “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’ll only be a matter of time until astronomers discover them.
This work was 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.”