Stellar ‘vampire’ finds love at first bite with fellow star

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An artist’s impression shows a “Vampire” star, consisting of an oblate star surrounded by a disk and a B-type star that has been stripped of its atmosphere (background), in this handout photo. Courtesy of ESO/L. Calcada/Handout via REUTERS THIS IMAGE WAS PROVIDED BY A THIRD PARTY. NO RESALE. NO ARCHIVES. COMPULSORY CREDIT.

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WASHINGTON, March 2 (Reuters) – Astronomers have a good look at what happens when a “vampire” star sucks out the outer layers of matter from a companion star, stripping that “bitten” victim into a simple stellar core.

Researchers said on Wednesday that data obtained using Chile-based European Southern Observatory (ESO) telescopes clarified the nature of a star system called HR 6819, showing that its two companion stars were not accompanied by a black hole as previously proposed.

Rather, the two stars exist as a binary system, gravitationally married in an orbit lasting about 40 days. While binary systems are common, what makes this one so unique is that it provided rare insight into the immediate consequences of so-called star vampirism.

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“What we mean by stellar vampirism is that a star has sucked out the outer material of another star,” said astronomer Abigail Frost of KU Leuven in Belgium, lead author of the research published in the Astronomy & Astrophysics journal.

“It can be done if the stars get close enough to each other and the gravitational pull of one star pulls material from the other star.”

Stars grow slowly as they age. Those in close-orbit two-binary systems—as in this case—can grow in size beyond a threshold at which their gravity can shield them from companion tug. It is therefore the star that grows the fastest that is the victim of vampirism.

“When this is done, inner areas of the star that have been ‘bitten off’ can be exposed, showing signatures of elements that we wouldn’t be easy to see otherwise,” Frost said.

This binary system is located about 1,000 light-years – the distance light travels in a year, 5.9 trillion miles (9.5 trillion km) – from Earth.

A 2020 study proposed that this system contained what would have been the closest black hole to Earth – in a three-way marriage with one star orbiting near the black hole and the other orbiting very far away – but new data indicate otherwise. Researchers who questioned the 2020 conclusion joined the new study along with scientists involved in this earlier work.

“It’s still a special system, and in my opinion even more so,” said ESO astronomer Thomas Rivinius, co-author of the new study and lead author of the 2020 research.

“Black holes exist in abundance, because once they form, they are permanent. Not so with what it turned out to be: it is a transitional stage short-lived – say about 10,000 years – in the evolution of a special double-star system.”

The system has two B-type stars, which are large, very hot, very bright, and bluish in color. For comparison, our sun is a G-type star, cooler and smaller than its B-type counterparts.

The rapid rotation seen in the “vampire” star may have been caused by material from the other star that had previously crashed into its surface. The “vampire” also now has a disc of material around it. The stars orbit quite close to each other – a third of the distance between Earth and the sun – but not as close as some binary stars.

The current mass of the “vampire” star is about four to five times that of the sun. The mass of the other star, originally perhaps five to six times the mass of the sun, may now be less than one solar mass.

With vampirism, most of the outer material sucked from the larger star ends up being incorporated into the thirsty companion.

“Since these outer layers were untouched by nuclear burning at the center of the original star, this material is ‘fresh,'” Rivinius said. “And so the other aspect of ‘vampirism’ is that this influx of fresh hydrogen actually ‘rejuvenates’ the star that receives it, making it appear bluer than it should be for its age.”

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Reporting by Will Dunham, editing by Rosalba O’Brien

Our standards: The Thomson Reuters Trust Principles.

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