Is HR 6819 a triple system containing a black hole?

(Image: ESO / L. Calcada)

A case of stellar vampirism has been discovered in the HR 6819 system so marked that in its day it was believed that there was a black hole in this system.

In 2020, a team led by astronomers from the European Southern Observatory (ESO) reported the discovery of the closest black hole to Earth, located just 1000 light-years away, in the HR 6819 system. But the results of their study were contested by other researchers, including an international team led from the University of Leuven in Belgium. In a new study, these two teams have teamed up to report that, in fact, there is no black hole in HR 6819, but rather a system of two stars involved in an act of “vampirism”, during a rare stage and brief of its evolution.

El estudio original sobre HR 6819 recibió mucha atención tanto por parte de la prensa como de la comunidad científica. Thomas Rivinius, astrónomo del ESO con sede en Chile y autor principal de ese estudio, no se sorprendió por la recepción por parte de la comunidad astronómica ante su descubrimiento del agujero negro. “No solo es normal, sino que debería ser común que los resultados sean revisados”, afirma, “y un resultado que llega a los titulares, aún más”.

Rivinius and his colleagues were convinced that the best explanation for the data they had, obtained with the MPG/ESO 2.2-metre Telescope, was that HR 6819 was a triple system, with a star orbiting a black hole every 40 days and a second star in a much wider orbit. But a study led by Julia Bodensteiner, then a doctoral student at the University of Leuven, proposed a different explanation for the same data: HR 6819 could also be a system with just two stars in a 40-day orbit and no black holes at all. . This alternative scenario would require one of the stars to be “stripped” of a large part of its mass, meaning that, at an earlier time, this mass had been “stolen” by another star.

“We had reached the limit of the existing data, so we had to resort to a different observation strategy to decide between the two scenarios proposed by the two teams,” says Abigail Frost, a researcher at the University of Leuven who led the new study. .

To solve the mystery, the two teams worked together to obtain new and sharper data from HR 6819. They used ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI) to do this. “The VLTI was the only facility that was able to provide us with the decisive data we needed to distinguish between the two explanations,” says Dietrich Baade, author of both the original HR 6819 study and the new study. Since there was no point in asking for the same observation twice, the two teams joined forces, allowing them to pool their resources and knowledge to explain the true nature of this system.

“The settings we were looking for were quite clear, very different and easily distinguishable with the right instrument,” says Rivinius. “We agreed that there were two light sources in the system, so the question was whether they orbited each other closely, as in the stripped stars scenario, or were they far apart from each other, as in the black hole scenario. “.

To distinguish between the two proposals, the teams used both the VLTI’s GRAVITY instrument and the Multi Unit Spectroscopic Explorer (MUSE) instrument, installed on ESO’s VLT.

“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 a third of the Earth-Sun distance,” says Frost. These data proved to be the final piece of the puzzle and allowed us to conclude that HR 6819 is a binary system without a black hole.

“Our best interpretation so far is that we picked up this binary system shortly after one of the stars had sucked the atmosphere out of its companion star. This is a common phenomenon in binary systems with their members very close to each other, sometimes called “stellar vampirism” in the press,” explains Bodensteiner, now a fellow at ESO in Germany and a co-author of the new study. “As the donor star was stripped of some of its material, the recipient star began to spin faster.”

“Capturing such a post-interaction phase is extremely difficult because it’s so short,” adds Frost. “This makes our findings on HR 6819 very exciting, as it is a perfect candidate to study how this vampirism affects the evolution of massive stars and, in turn, the formation of associated phenomena, including gravitational waves.” and violent supernova explosions.