Closest Black Hole To Earth Is Missing, In Its Place Rare 'Stellar Vampire'

Astronomers have discovered that a star system believed to host the closest black hole to Earth is actually comprised of just two stars. But, while a black hole may not lurk in the system, something rarer and as exciting does: a so-called "stellar vampire" that has fed on material from its companion star.

In 2020, the announcement of a system containing two stars and a black hole just 1,000 light-years from Earth made headlines across the globe, representing the closest cosmic object of this type to Earth.

But, the findings were challenged by other researchers with both teams joining forces to discover that rather than a triple system, HD 6819 is actually a "vampire" two-star system: stars orbiting each other in a rare and short-lived stage of their evolution, with one weak and emaciated after being fed upon.

Now, a European Southern Observatory fellow, Julia Bodensteiner, has led the follow-up research as a Ph.D. student at KU Leuven, Belgium. She told Newsweek: "According to our current best interpretation, we caught the HR 6819 binary system in a phase just after the two companion stars have interacted: one star has transferred material to its companion star and was stripped of its outer envelope.

"The other star received at least some of this material and was spun up, just like a spinning top. The phase we are currently observing is very short in comparison to the lifetime of the entire system, which makes it very difficult to catch by observations."

Bodensteiner said astronomers call a binary system a vampire system if the two stars are currently interacting. She continued: "That means that the two stars are so close to each outer that one star basically sucks the outer envelope of the second star, similarly to vampires which are said to suck blood."

The stars in HD 6819 are now believed to circle each other with a 40-day orbit with one star having stripped the other's upper layers of material. This means that this donor star had at some point in the past lost a huge amount of mass.

Abigail Frost is a researcher at KU Leuven in Belgium who was part of the team. She explained to Newsweek this is an important finding: "Stars like those in HR 6819 are very important players in the Universe, creating lots of complicated materials like iron, oxygen, and more that we find on Earth.

"So, in a way, by better understanding how the materials in these stars are affected by interactions, we can better understand the processes that eventually led to the formation of our own home in the Universe."

Joining Forces To Solve A Black Hole Mystery

The lead researcher of the team that believed they had discovered a triple star system with a black hole, ESO astronomer Thomas Rivinius, worked with Bodensteiner and her researchers to solve the mystery, despite disagreeing about the nature of the system.

Rivinius told Newsweek: "We basically had two competing scenarios. We knew there were two sources of light in the system, so the question was what relation they are in?

"Are they orbiting each other, which implies one of them must be a very unusual star, caught in a brief state of transition after an interaction, or far apart, in which case there must be a third, dark body, purely supplying a source of gravity for one of the two luminous stars to orbit around. Which means a black hole."

The key to solving this puzzle was obtaining a clearer picture of HD 6819, which the joint team was able to do by using ESO's Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI).

The right instrument meant that the scenarios became much clearer and more distinguishable. Picking out two sources of light, the team then had to determine if they were far apart, as they would be in the black hole triple system, or if they are close together in the alternative two-star "vampire system."

Rivinius continued: "With the capabilities of the VLT and VLTI we could exclude the wide scenario, and even begin to measure their path, how they orbit each other closely."

This led to the discovery there was no black hole in the system, which instead is composed of two stars, one of which fed on the other.

No Black Hole But No Less Exciting

Dietrich Baade was an author of the original work that suggested a triple system, and worked on this new research led by Bodensteiner which posits a binary "vampire system" published in the journal Astronomy & Astrophysics.

Baade told Newsweek the lack of a black hole in HD 6819 doesn't make it any less exciting a subject of study. Quite the opposite, as he argued that: "While a black hole may appear exotic and exciting, the star in HR 6819 that has lost most of its mass is not any less exciting.

"It is very common for the components of close double stars to exchange mass as has happened in HR 6819. But after the mass-transfer period, the mass doners shrink quickly to become a very small and hot subdwarf."

Baade adds that in HR 6819, the mass loss happened so recently that the loser has not yet substantially contracted. He continues: "Stellar vampires do not suck blood but gas because stars are gas balls. Like other vampires, they can substantially weaken their victims.

"This is a small window of opportunity to observe a star's interior that was totally invisible until the star's outer layers were stripped off."

Bodensteiner added: "Observing this short-lived and therefore rare phase just after the binary interaction has occurred can provide us with direct constraints on the history of the system. We can for example get an idea of how much of the material lost by one star is received by the other star.

"I find it fascinating that some binary stars do not merely orbit around their companion star. If they are close enough, they can also interact with each other, which can drastically change both the further evolution as well as the nature of both stars."

Vampire Star
An illustration of HR 6819. The system was once believed to be a triple system with a black hole and two stars; but, the black hole is missing and in its place is a rare vampire star feeding on its companion. ESO/L. Calçada

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