Back in 2020, researchers based at the European Southern Observatory (ESO) made an exciting announcement – they had found the closest ever black hole to Earth.
It was, according to the team led by Thomas Rivinius, lurking just 1,000 light-years away in a triple system dubbed HR 6819. According to the ESO team, it comprised one star that was orbiting a black hole in a tight, 40-day orbit and another one much further out.
However, not everyone was convinced. Particularly Julia Bodensteiner, who was then a PhD student based at KU Leuven, Belgium. She thought it was more likely that HR 6819 was a two-star system in which one of the stars was ‘feeding’ on material from the other – a so-called vampire system.
To get to the bottom of the mystery, the two teams joined forces to gather more data on the rare system using ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI).
“The scenarios we were looking for were rather clear, very different and easily distinguishable with the right instrument,” said 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.”
After careful analysis, they were able to rule out the possibility of there being a star out in a wider orbit and so concluded that HR 6819 is a binary system.
“Our best interpretation so far is that we caught this binary system in a moment shortly after one of the stars had sucked the atmosphere off its companion star. This is a common phenomenon in close binary systems, sometimes referred to as ‘stellar vampirism’ in the press,” said Bodensteiner, now a fellow at ESO in Germany.
“While the donor star was stripped of some of its material, the recipient star began to spin more rapidly.”
The team plan further observations of HR 6819 in the hope that they will be able to shed further light on the properties and evolution of vampirism in other binary star systems as well as other phenomena associated with them such as gravitational waves and supernovae.
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