Spooky stars: phantom black hole is actually a “stellar vampire”

A star system previously described as the site of the closest black hole to Earth does not actually contain a black hole, a new study has reported.

Instead, the HR 6819 system is home to a rare phenomenon known as a vampire star.

Back in 2020, a team of astronomers based at the European Southern Observatory (ESO) published a paper on HR 6819. Using observations from the MPG/ESO 2.2-metre telescope, they proposed that HR 6819 was a triple system containing a black hole, one star orbiting the black hole, and a second star in a wider orbit. The black hole in HR 6819 would have been the closest to Earth yet observed.

However, another team based at KU Leuven in Belgium believed that the observations could equally be explained by a binary system, with two stars in orbits of similar lengths and no black hole.

For this alternative explanation to be correct, one of the stars would have to be “stripped” – meaning that it had lost a large proportion of its mass to the other at some point in the past.

In the best scientific spirit, the two teams decided to work together to seek the truth.

“We had reached the limit of the existing data,” explains Abigail Frost, a researcher at KU Leuven and leader of the new study. “So we had to turn to a different observational strategy to decide between the two scenarios proposed by the two teams.”

“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,” says Thomas Rivinius, a lead author on the original ESO paper.

The debate was clinched by data collected using the GRAVITY and Multi Unit Spectroscopic Explorer (MUSE) instruments on ESO’s Very Large Telescope Interferometer (VLTI).

“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 one-third of the distance between the Earth and the Sun,” 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 with no black hole.” 

However, while they may have lost a black hole, the researchers believe they have gained a rare sighting of a spooky astronomical occurrence.

In binary systems where two stars are close together, it’s not uncommon for one star to “suck” away the atmosphere of the other – a phenomenon sometimes called “stellar vampirism”. The researchers believe they may have observed the immediate aftermath of a stellar vampire attack in HR 6819.

“While the donor star was stripped of some of its material, the recipient star began to spin more rapidly,” says Julia Bodensteiner, who led the study proposing the stripped-star scenario as a PhD student at KU Leuven and is now a research fellow at ESO.

“Catching such a post-interaction phase is extremely difficult as it is so short,” says 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, and in turn the formation of their associated phenomena including gravitational waves and violent supernova explosions.”

Far from leading to acrimony, the original debate about HR 6819 has nurtured scientific understanding and the formation of a new collaboration between the astronomers.

“Not only is it normal, but it should be that results are scrutinised,” says Rivinius.

Meanwhile, the search for Earth’s closest black hole continues.