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A stellar vampire mimics a black hole

4 March 2022

The star system thought to host the Sun’s nearest black hole neighbor is actually just a binary pair at an unusual evolutionary stage.

Artist’s impression of HR 6819
An artist’s rendition of a so-called vampire star, foreground, after stripping off its companion’s atmosphere. Credit: ESO/L. Calҫada

In May 2020 a team of astronomers reported the discovery of a black hole just 1000 light-years from Earth. It was the closest found to date, so close that on a clear night the two stars orbiting it could be seen with the naked eye. The researchers, led by Thomas Rivinius from the European Southern Observatory (ESO), hoped that the system could become a test bed for studying nonaccreting stellar-mass black holes.

Later that year, however, Julia Bodensteiner and coworkers at KU Leuven in Belgium proposed an alternative explanation for the ESO group’s observations. The stellar spectra that Rivinius and colleagues had used to draw their conclusion were also consistent with a pair of stars in which one had recently stripped the other of its atmosphere. Because the recipient acquires angular momentum from the added material, it rapidly rotates. Such stellar vampirism is not uncommon, but catching a binary before it has had a chance to further evolve is.

The ESO and Leuven groups teamed up to figure out which explanation was correct. Their new study, led by Abigail Frost (KU Leuven), concludes that the system has just two bodies. One is a bright, blue, B-type giant that’s been stripped of its atmosphere. Its accreting companion, a Be-type star, is similarly bright but with distinct spectral features. The results rule out a central black hole.

Existing observations lacked sufficient resolution to distinguish between the two scenarios, so the researchers turned to the Very Large Telescope in Chile. They used its multiunit spectroscopic explorer instrument to collect spectra with finer spatial resolution. The new observations showed both stars residing in a close orbit, consistent with the two-body picture, rather than the Be star circling in a wide orbit, as it would have been in the three-body picture.

Images from the Very Large Telescope Interferometer’s GRAVITY instrument were also consistent with a close orbit. They showed a small separation of about 1 milliarcsecond between the B and Be stars; the black hole scenario predicted an approximately 120 mas separation.

Despite its lack of a black hole, the stellar system could inform models of binary-star evolution, Be-type star formation, and accretion in binary pairs. (A. J. Frost et al., Astron. Astrophys. 659, L3, 2022.)

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