No invisible third party: The black hole closest to Earth, which was only discovered in 2020, is in fact not one, as new observations have now revealed. Instead, a case of “stellar vampirism” in the binary star system HR 6819 appears to be producing the misleading signal: one of the stars has sucked almost the entire envelope off its partner. As a result, it rotates unusually fast and the material loosely surrounding it creates the unclear features in the light spectrum.
Stellar black holes are the relics of massive stars that have exploded in a supernova. In this respect, it is reasonable to assume that there are such star relics in our closer cosmic environment. The problem, however, is that when the black holes are not actively sucking in matter and thereby releasing radiation, they are invisible. Such are correspondingly difficult silent black holes to prove.
Hidden companion – or not?
But in May 2020, astronomers led by Thomas Rivinius from the European Southern Observatory (ESO) reported a “Manhunt success“: In the HR 6819 system, only 1,000 light-years away, they had found not only two normal stars but also indications of an invisible third party. According to the spectral data, one of the stars there orbits a four-solar-mass black hole in 40 days, and a second star orbits both at a greater distance, according to the team’s interpretation.
However, a little later, further observations by a team from the Catholic University of Leuven led by Julia Bodensteiner raised doubts about this interpretation. Their data suggested that HR 6819 could also consist of just two closely orbiting stars – without a dormant black hole partner. The abnormalities in the light spectrum would then have to be due to a rapidly rotating Be star – a star surrounded by loose, strongly radiating shell remnants.
New search for clues
The problem, however: “We had reached the limit of the existing data, so we had to use a different observation strategy to decide between the two scenarios proposed by the two teams,” explains first author Abigail Frost from KU Leuven. Both teams of astronomers therefore teamed up to re-examine HR 6819 using the MUSE and GRAVITY spectrographs at the Very Large Telescope of the European Southern Observatory in Chile.
Only the four coupled telescopes of the observatory provided the necessary resolution to image the features of the multiple system. “The VLTI was the only facility that could give us the crucial data we needed to distinguish between the two explanations,” says co-author Dietrich Baade from ESO.
Rivinius adds: “The scenarios we were looking for were pretty clear, very different and easy to distinguish with the right tool. We agreed that there are two light sources in the system. So the question was whether they orbit each other closely, as in the stripped star scenario, or far apart, as in the black hole scenario.”
Stellar vampire instead of black hole
The new observations revealed: There does not appear to be a black hole in the HR 6819 system. “MUSE confirmed that there was no bright companion in another 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 allowed us to conclude that HR 6819 is a binary star system without a black hole.”
But how can the anomalies in the spectral data be explained? As previously suspected by Bodensteiner and her team, one of the two partner stars in this system is apparently a “vampire star”. He sucked off large parts of the shell from his partner. “It’s extremely difficult to capture such a post-exchange phase because it’s so brief,” explains Frost.
The manhunt continues
Even if there is no silent black hole hidden in this star system, it is still an exciting case from an astronomical point of view: “This makes our results for HR 6819 very exciting, because it is a perfect candidate to study how this vampirism affects the development of massive stars and thus also the emergence of the associated phenomena such as gravitational waves and violent supernova explosions,” says Frost.
The astronomers therefore plan to continue observing HR 6819 to better understand the evolution of this stellar vampirism. But they are also not giving up on the search for nearby black holes: “Estimates of magnitudes indicate that there are tens to hundreds of millions of black holes in the Milky Way alone,” says Baade. It is only a matter of time before one of them is also found in our cosmic environment. (Astronomy & Astrophysics, 2022; doi: 10.1051/0004-6361/202143004)
Quelle: European Southern Observatory (ESO)