Artist's impression of the newly-discovered vampire star (foreground) and the companion star (background) that has been stripped of its atmosphere by the vampire. Credit: ESO / L. Calçada.

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.

View of the area of sky where the system HR 6819 is located. The two stars in HR 6819 can be viewed from the southern hemisphere on a dark, clear night without binoculars or a telescope. Credit: ESO / Digitized Sky Survey 2. Acknowledgement: Davide De Martin.

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.

Quelle: COSMOS

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Update: 14.03.2022

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Earth’s purported ‘nearest black hole’ isn’t a black hole

A contested multiple star system is a missing link in stellar evolution

The star system HR 6819 (illustrated) hosts no black holes, as previously reported, but instead two blue stars, one that recently siphoned gas off the other.

The nearest black hole to Earth isn’t a black hole at all. Instead, what scientists thought was a stellar triplet — two stars and a black hole — is actually a pair of stars caught in a unique stage of evolution.

In May 2020, a team of astronomers reported that the star system HR 6819 was probably made up of a bright, massive star locked in a tight, 40-day orbit with a nonfeeding, invisible black hole plus a second star orbiting farther away. At about 1,000 light-years from Earth, that would make this black hole the nearest to us (SN: 5/6/20). But over the following months, other teams analyzed the same data and came to a different conclusion: The system hosts only two stars and no black hole.

Now, the original team and one of the follow-up teams have joined forces and looked at HR 6819 with more powerful telescopes that collect a different type of data. The new data can make out finer details on the sky, allowing the astronomers to definitively see how many objects are in the system and what type of objects they are, the teams report in the March Astronomy & Astrophysics.

“Ultimately, it was the binary system that best explains everything,” says astronomer Abigail Frost of KU Leuven in Belgium.

Previous observations of HR 6819 showed it as a unit, so astronomers couldn’t differentiate the objects in the system nor their masses. To nail down HR 6819’s true nature, Frost and colleagues turned to the Very Large Telescope Array, a network of four interconnected telescopes in Chile that can essentially see the separate stars.

“It allowed us to disentangle that original signal definitively, which is really important to determine how many stars were in it, and whether one of them was a black hole,” Frost says.

The scientists think one of the stars is a massive bright blue star that has been siphoning material from its companion star’s bloated atmosphere. That companion star now has little gaseous atmosphere left. “It’s already gone through its main life, but because the outside has been stripped off, and you only see the exposed core, it has similar temperature and luminosity and radius to a young star,” says Kareem El-Badry, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. El-Badry was not involved in the new study, but he suggested in 2021 that HR 6819 is a binary system.

This siphoned star’s core color and brightness could fool astronomers looking at the older data into thinking it was a young star with 10 times as much mass. It originally appeared as though this star was orbiting something massive but invisible — a black hole.

Once the researchers unraveled the system’s details, they realized this system is a unique one, showing astronomers a phase not seen before among systems with massive stars. “It is a missing link in binary star evolution,” says astrophysicist Maxwell Moe of the University of Arizona in Tucson, who was also not part of the new study.

Astronomers for years have seen binary systems where one star is actively pulling gas off the other, and they’ve seen systems where the donor star is just a naked stellar core. But in HR 6819, the donor star has stopped giving mass to the other. “It still has a little bit of envelope left but is quickly contracting, evolving to become a remnant core,” Moe says.

Frost and her colleagues are using the Very Large Telescope Array to monitor HR 6819 over a year to track precisely how the stars are moving. “We want to really understand how the individual stars in the system are ticking,” she says. The team will then use that information in computer simulations of binary star evolution. “[It’s] exciting to now have a system that we can use as kind of a cornerstone to investigate this in more detail,” Frost says.

Even though HR 6819 doesn’t have the nearest black hole to Earth, it appears to have something more useful to astronomers.

Quelle: ScienceNews