‘Closest black hole system’ to Earth doesn’t actually have one

Written by The Daily Mail on March 4, 2022. Posted in Current News

When it was discovered in 2020, two scenarios were proposed for the discovery of the unusual HR 6819 system, one with, and one without a black hole.

The idea of a black hole just 1,000 light years away, within our stellar neighbourhood, hit the headlines at the time of the discovery and was presented as the main finding – with the other scenario not being widely reported.

However, a new study, by researchers from KU Leuven in Belgium, took new measurements of the system, to see which scenario was most likely to be true.

They pointed the European Southern Observatory Very Large Telescope (ESO, VLT), and the VLT Interferometer at the star system, as this instrument can provide decisive data required to distinguish between the two explanations.

All the evidence pointed to a ‘vampire’ star system, finding two stars in a very close orbit, with one ripping material from the other.

There was no evidence of a distant orbiting star, as would be required by the black hole theory.

Thomas Rivinius, a Chile-based ESO astronomer and lead author on the original black hole paper, was not surprised by the astronomy community’s reception.

‘Not only is it normal, but it should be that results are scrutinised,’ he said, adding that it was ‘even more so’ important for results that ‘make the headlines’.

Rivinius and his colleagues were convinced that the best explanation for the data they had was that HR 6819 was a triple system, with one star orbiting a black hole every 40 days and a second star in a much wider orbit.

But a study led by Julia Bodensteiner, then a PhD student at KU Leuven, Belgium, proposed a different explanation for the same data.

She suggested that HR 6819 could also be a system with only two stars.

Each of the stars would be found in a very close 40-day orbit, with no black hole at all.

This alternative scenario would require one of the stars to be ‘stripped’, meaning that, at an earlier time, it had lost a large fraction of its mass to the other star.

‘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 KU Leuven researcher Abigail Frost, who led the new study

The researchers turned to the Very Large Telescope, and in particular the Interferometer on the VLT, known as the VLTAI, to gather detailed observations.

‘The VLTI was the only facility that would give us the decisive data we needed to distinguish between the two explanations,’ says Dietrich Baade, author on both the original HR 6819 study and the new paper.

Working together, the new team worked with Baade and colleagues, so they only needed to request time on the VLT once.

‘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.’

They used the GRAVITY and MUSE instruments on the VLT to study the star system, and pin down the distance each star is from its partners, finding no evidence of a bright companion star in a wide orbit – as the black hole model predicted.

They also found two bright objects in a close orbit, as suggested by the ‘vampire’ star model.

‘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,’ said 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.’

Bodensteiner said the best interpretation is that they caught this binary system in a moment shortly after one of the stars sucked the atmosphere off its companion.

‘This is a common phenomenon in close binary systems, sometimes referred to as “stellar vampirism” in the press,’ he explained.

‘While the donor star was stripped of some of its material, the recipient star began to spin more rapidly.’

Catching this sort of ‘post-interaction phase’ of a binary star system is difficult, as this point in the life of the two stars is very short.

‘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,’ said Frost.

The newly formed Leuven-ESO joint team now plans to monitor HR 6819 more closely using the VLTI’s GRAVITY instrument.

The researchers will conduct a joint study of the system over time, to better understand its evolution, constrain its properties, and use that knowledge to learn more about other binary systems.

They’re also optimistic that they could find a black hole in our stellar neighbourhood at some point in the future.

‘Stellar-mass black holes remain very elusive owing to their nature,’ says Rivinius. ‘But order-of-magnitude estimates suggest there are tens to hundreds of millions of black holes in the Milky Way alone,’ Baade adds.

The findings have been published in the journal Astronomy & Astrophysics.

See more here: dailymail.co.uk

Header image: European Space Observatory

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