Surprise! The "closest black hole" is actually just "stellar vampirism"

Sometimes mistakes can lead to the biggest discoveries
Brad Bergan
A computer-generated depiction of the binary stars.ESO / L. Calçada

When Astronomers at the European Southern Observatory came across a massive energy source a mere 1,000 light-years away, their finding surged through the scientific community. 

It looked like we'd discovered the closest black hole ever. We might be able to peer into the dark abyss with only a 1,000-year lag.

But that incredible discovery also came with some skepticism. When it comes to science that holds the potential to reshape our view of the universe, the utmost scrutiny is required.

"Not only is it normal, but it should be that results are scrutinized," explains Thomas Rivinius, a Chile-based astronomer with the ESO.

That scrutinizing second look is how a pair of researchers came to propose an explanation that breaks with that initial black hole idea.

The HR 6819 system where the alleged black hole was located might have no black hole at all.

Scientists thought that a swirling volume of matter was circling down the drain of a black hole, but this could simply be "stripped" matter from a star that lost a major portion of its mass to another star. This 180-degree finding is explained in a study published Wednesday in the journal Astronomy and Astrophysics.

The corrective study might help explain two wonders of the universe: gravitational waves and supernova explosions.

Astronomers join forces to discover "stellar vampirism" where a black hole was thought to lurk

"We had reached the limit of the existing data, so we had to turn to a different observational strategy to decide between the two scenarios proposed by the two teams," says Abigail Frost, a researcher at KU Leuven who led the new study.

To unlock the secrets of this perplexing system, both science teams — the one from the initial, black hole extravaganza, and the later one with an alternative theory in mind — collaborated to develop new, more high-definition data of HR 6819.

This was accomplished with the ESO's Very Large Telescope, in addition to the Very Large Telescope Interferometer (VLTI), both located in Chile.

"The VLTI was the only facility that would give us the decisive data we needed to distinguish between the two explanations," said an author of both the new study, and the initial black hole-positing study on HR 6819.

This teamwork was essential in the struggle to unlock the true nature of the HR 6819 system.

"The scenarios we were looking for were rather clear, very different, and easily distinguishable with the right instrument," says Rivinius. "We agreed that there were two sources of light in the system, so the question was whether they orbit each other closely, as in the stripped-star scenario, or are far apart from each other, as in the black hole scenario."

Employing a dual-strategy, the intrepid collective of astronomers leveraged both the Multi-Unit Spectroscopic Explorer (MUSE) instrument (which is on the ESO's VLT) and the VLTI's GRAVITY instrument.

MUSE enabled the astronomers to establish that there was no bright companion in a wider orbit, but the high spatial resolution of GRAVITY resolved "two bright sources separated by only one-third of the distance between the Earth and the sun," explained Frost.

One mistaken black hole observation could help unlock cosmic mysteries

"These data proved to be the final piece of the puzzle, and allowed us to conclude that HR 6819 is a binary system with no black hole," added Frost. The mega-team of astronomers then made a calculated guess at the moment they'd captured with the telescopic instruments, and suspect that they caught the system in the moment "shortly after one of the stars had sucked the atmosphere off its companion star," says Julia Bodensteiner, an ESO fellow in Germany, who's also an author of the new study.

"This is a common phenomenon in close binary systems, sometimes referred to as 'stellar vampirism' in the press," adds Bodensteiner. When the "donor" star was being stripped of some of its material — the recipient star started to pirouette, faster and faster.

In other words, the superteam of astronomers caught the pair of interlocked stars in a very rare moment — one that's "extremely difficult" to capture, explains Frost.

"This makes our findings for HR 6819 very exciting, as it presents a perfect candidate to study how this vampirism affects the evolution of massive stars," which could help unlock the mysteries of both gravitational waves, and supernova explosions.

It's rare for a mistake to lead to a new discovery, and even rarer still for a mistake to be followed-up by something that could fundamentally transform our grasp of the entire universe. But that's what happened.

Study Abstract:

Two scenarios have been proposed to match the existing observational constraints of the object HR 6819. The system could consist of a close inner B-type giant + black hole (BH) binary with an additional Be companion on a wide orbit. Alternatively, it could be a binary composed of a stripped B star and a Be star in a close orbit. Both scenarios make HR 6819 a cornerstone object, either as the closest stellar BH to Earth, or as an example of an important transitional, non-equilibrium phase for Be stars with solid evidence to back its nature. We aim to distinguish between the two scenarios for HR 6819. Both models predict two luminous stars but with very different angular separations and orbital motions. Therefore, the presence of bright sources in the 1-100 milli-arcsec (mas) regime is a key diagnostic to determine the nature of the HR 6819 system. We obtained new high-angular resolution data with VLT/MUSE and VLTI/GRAVITY of HR 6819. The MUSE data are sensitive to bright companions at large scales, whilst the interferometric GRAVITY data are sensitive down to separations of order mas scales and large magnitude differences. The MUSE observations reveal no bright companion at large separations and the GRAVITY observations indicate the presence of a stellar companion at an angular separation of ∼ 1.2 mas, moving on the plane of the sky over a time scale compatible with the known spectroscopic 40 day period. We conclude that HR 6819 is a binary system and that no BH is present in the system. The unique nature of HR 6819, and its proximity to Earth make it an ideal system to quantitatively characterize the immediate outcome of binary interaction and probe how Be stars form.

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