May 26, 2022

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HD 6819

The black hole closest to Earth is not – the alleged signal of the remains of a star is due to a stellar vampire

No invisible third party: The black hole closest to Earth, discovered only in 2020, isn’t actually a third party, new observations have now revealed. Instead, the “stellar vampire” state in the binary star system HR 6819 appears to produce a misleading signal: one star has sucked the envelope almost completely from its partner. As a result, it rotates at an unusual speed and the material around it loosely creates blurs in the light spectrum.

Stellar black holes are the remnants of massive stars that exploded in a supernova. In this regard, it is reasonable to assume that such stellar traces exist in our closest cosmic environment. However, the problem is that when black holes do not actively absorb matter and thus release radiation, they are invisible. On the other hand, this is difficult silent black holes to prove.

Invisible companion – or not?

But in May 2020, astronomers led by Thomas Rivinius of the European Southern Observatory (ESO) reported that “stalking success”: In the HR 6819 system, which is only 1,000 light-years away, they found not only two ordinary stars, but also indicators of an invisible third party. According to spectroscopic data, one of the stars there orbits a black hole of four solar masses within 40 days , and a second star orbits at a greater distance, according to the team’s explanation.

However, a little later, additional observations made by a team from the Catholic University of Leuven led by Julia Budensteiner cast doubts about this interpretation. Their data indicates that HR 6819 could also consist of two stars orbiting each other – without a black hole’s sleeping partner. Then, the anomalies in the light spectrum should be caused by the rapid rotation of a Be – a star surrounded by the remnants of a loose, strongly radiant crust.

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New search for clues

The problem, though: “We reached the limit of the existing data, so we had to use a different observational strategy to decide between the two scenarios proposed by the two teams,” explains first author Abigail Frost of KU Leuven. So both teams of astronomers collaborated to re-examine HR 6819 using the MUSE and GRAVITY spectrometers at the European Southern Observatory’s Very Large Telescope in Chile.

Only the observatory’s four paired telescopes provided the resolution needed to depict the multiple system features. “The VLTI was the only facility that could provide us with the critical data we needed to distinguish between the two interpretations,” says co-author Dietrich Bade from ESO.

Rivinius adds: “The scenarios we were looking at were very straightforward, very different and easy to distinguish with the right tool. We agreed that there were two light sources in the system. So the question was whether they were orbiting closely around each other, as in the naked star scenario, or far away. , as in the black hole scenario.

An excellent vampire instead of a black hole

New observations revealed: There does not appear to be a black hole in the HR 6819 system. “MUSE confirmed the absence of a bright companion in another orbit, while GRAVITY’s high spatial resolution was able to identify two bright sources separated by a distance of a third of the distance from Earth,” says Frost. And the sun.” “These data allowed us to conclude that HR 6819 is a binary star system without a black hole.”

But how can aberrations in the spectral data be explained? As Bodensteiner and her team previously suspected, one of the two partner stars in this system appears to be a “vampire star”. He sucked large portions of the cochlea from his partner. “It is very difficult to capture such a stage after the exchange because it is so short,” Frost explains.

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The chase 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, as it is an ideal candidate for studying how this vampire influences the evolution of massive stars and thus the emergence of associated phenomena They are like gravitational waves and violent supernova explosions,” says Frost.

So the astronomers plan to continue monitoring HR 6819 to better understand the stellar vampire evolution. But they’re also not giving up on the search for nearby black holes: “Size estimates suggest there are tens to hundreds of millions of black holes in the Milky Way alone,” Badie says. It is only a matter of time before one of them is found in our cosmic environment. (Astronomy and Astrophysics, 2022; doi: 10.1051/0004-6361/202143004)

Source: European Southern Observatory (ESO)