“Vampire” Two-Star System: Creates the Illusion of a Black Hole Closest to Earth

HR-6819

 

The first black hole to be discovered was Cygnus X-1 in 1964. It comprises a black hole and a blue supergiant star orbiting each other. The black hole is accreting from the wind of the blue supergiant, forming a hot disk around the black hole that emits powerful X-rays. The mass of the invisible  companion is calculated to be 8-10 solar masses, much too large to be a neutron star.

Fast forward to 2020: a team led by European Southern Observatory (ESO) astronomers reported the closest black hole to Earth, located just 1,000 light-years away in the HR 6819 system. The results of their study, however,  were contested by an international team based at KU Leuven, Belgium. These two teams collaborated to report that there is in fact no black hole in HR 6819, which is instead a “vampire” two-star system in a rare and short-lived stage of its evolution.

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Science Does Its Job!

The original study on HR 6819 received significant attention from both the press and scientists. Thomas Rivinius, a Chile-based ESO astronomer and lead author,, was not surprised by the astronomy community’s reception to their discovery of the black hole. “Not only is it normal, but it should be that results are scrutinized,” he says, “and a result that makes the headlines even more so.”

Rivinius and his colleagues were convinced that the best explanation for the data they had, obtained with the MPG/ESO 2.2-metre telescope, 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: HR 6819 could also be a system with only two stars on a 40-day orbit and 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.

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“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 published in Astronomy & Astrophysics.

Tale of Two Scenarios

To solve the mystery, the two teams worked together to obtain new, sharper data of HR 6819 using ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI). “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 Astronomy & Astrophysics paper. Since it made no sense to ask for the same observation twice, the two teams joined forces, which allowed them to pool their resources and knowledge to find the true nature of this system.

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“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.”

VLT Technology: GRAVITY and MUSE

To distinguish between the two proposals, the astronomers used both the VLTI’s GRAVITY instrument and the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO’s VLT.

“GRAVITY and MUSE were both crucial to converging on our result,” Dr. Abigail Frost wrote in an email to The Daily Galaxy. “Two scenarios existed to explain spectral observations of HR 6819, she explained. “1) it was a triple system with a black hole and 2) it was a binary system with no black hole. For scenario 1, we would expect to see a bright star on a wide orbit around a close binary composed of an invisible source (the black hole) and another bright star.

“For scenario 2,” Frost explained in her email, “there was no black hole and the two bright stars in the system were orbiting each other closely instead and their light signatures were irregular due to a previous interaction. We couldn’t definitively choose between the two scenarios without detecting the stars and where they were more directly. This is what GRAVITY and MUSE let us do. With MUSE images we confirmed that no bright star was present on a wide orbit and with GRAVITY we confirmed that the two bright stars were orbiting each other on a short orbit. Furthermore, GRAVITY let us work out that the brighter star in the system was likely the Be star by seeing evidence of this discs rotation.

Binary System with No Black Hole

“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,” says Frost. 

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“The two instruments, MUSE and GRAVITY, have complementary capabilities, the combination of which proved decisive for the success of our project,” wrote ESO astrophysicist Dietrich Baade in an email to The Daily Galaxy. “In the triple system with BH”, he explained, “the two stars could still appear very close to one another.  For instance, Venus and Jupiter can momentarily be very close in the sky but this does not mean that they are also physically close to each other.  When both planets move on in their orbits, they separate in the sky.  That is, we might have hit the two stars in the triple stars while they appeared close,  just by chance. We called this the “bad-luck” problem.”

The “Bad-Luck” Problem

“To overcome the bad-luck problem, we used GRAVITY on the VLTI”, Baade continued in his email. “While the MUSE instrument is fed by just one (eight-meter) unit telescope of the VLT, GRAVITY can combine the light from four telescopes.  We employed the 4 so-called auxiliary telescopes (ATs), each of which has a diameter of 1.8 m.  The ATs can be moved on rails.  For our observations, the ATs were separated by between nearly 60 meters and more than 130 meters, which is the maximum possible.  GRAVITY superposes the waves from the telescopes in a very sophisticated way.  This does not produce an image but so-called fringes.  The separation between these fringes corresponds to a path difference between the light coming from one telescope and that from another one by one wavelength.  From the fringe pattern, the separation of two objects can be measured with a resolution of better than 0.001 arcsec (1 mas).” 

“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,” explains Bodensteiner, now a fellow at ESO in Germany and an author on the new study. “While the donor star was stripped of some of its material, the recipient star began to spin more rapidly.”

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“Catching such a post-interaction phase is extremely difficult as it is so short,” adds 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.”

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.

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Search for Stellar-Mass Black Holes Continues

As for the search for black holes, the team remains optimistic. “Stellar-mass black holes, observed as a hypernova explosion or as a gamma ray burst, 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. It is just a matter of time until astronomers discover them. Physics World reported on 03 February 2022 the first unambiguous detection and mass measurement of an isolated stellar mass black hole wandering through the Milky Way, likely the remnant of a single massive (> 20 solar masses) star, which reached the end of its life and collapsed into a black hole. 

This research was presented in the paper “HR 6819 is a binary system with no black hole: Revisiting the source with infrared interferometry and optical integral field spectroscopy” (DOI: 10.1051/0004-6361/202143004) to appear in Astronomy & Astrophysics. 

Image at top of the page: artist’s impression of HR 6819 Credit: ESO/L. Calçada

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Dietrich Baade, Abigail Frost and  ESO

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