Astronomers combined forces to confirm that a black hole proposed to lie a mere 1,000 light-years away isn’t really there.
A contested candidate for the closest black hole to the solar system has proven to be a mirage, an international team of astronomers has concluded.
The putative black hole lay in the stellar binary HR 6819, some 1,000 light-years away in the constellation Telescopium. HR 6819 appears to be a single, bright star, but its spectrum reveals that there are in fact two stars present: a strange type of rapidly spinning, bright bluish star skirted by a disk of hot gas, called a Be star, and a second, also bluish B-type star hidden in the other star’s light.
In 2020, Thomas Rivinius (European Southern Observatory, Chile) and colleagues teased apart the binary’s spectra to discover that the B-type star was moving through a 40-day orbit. The Be star showed no clear signs of moving, so a third, invisible companion was likely to blame, the team proposed, perhaps a black hole.
But as we reported at the time, other astronomers were skeptical. Subsequent analyses by other teams suggested that the secondary star was a stripped-down version of its former self, complicating the earlier calculations, and even found hints of the 40-day orbit in the Be star’s light, suggesting the larger star was the cause of the B star’s movement.
To settle the debate, Rivinius’s team and some of the skeptics joined forces. Using imaging and spectroscopic instruments at the Very Large Telescope facility in Chile, the team was able to split the stars apart and confirm that they lie only one-third of the Earth-Sun distance from each other, not at the much wider separation that would require a close-in, third object to explain the 40-day orbit. Abigail Frost (KU Leuven, Belgium) heads up the team’s report in the March Astronomy & Astrophysics.
Instead of containing a black hole, the astronomers explain, HR 6819 is a unique kind of stellar system. They think the Be star has just sucked the atmosphere off its companion star, explaining the gas skirt the Be star wears. It’s good evidence that, as suspected, Be stars have spun themselves up and vested themselves in gas thanks to recent interactions with another star. It’s also a rare chance to study this “stellar vampirism” in action.
A. J. Frost et al. “HR 6819 Is a Binary System with No Black Hole: Revisiting the Source with Infrared Interferometry and Optical Integral Field Spectroscopy.” Astronomy & Astrophysics, March 2022.