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Doubts about the black hole closest to Earth

A few months ago, scientists announced the indirect detection of the black hole closest to Earth. But another team now suggests a different explanation for this stellar puzzle.

From the earliest spectra of the HR 6819 star system, scientists identified this source as a star Be sparkly early type: a hot star with emission lines, probably due to the accumulation of a disk of circumstellar material. However, as the ability to resolve details in stellar spectra advanced, a more complicated picture emerged.

Studies conducted in the decade of 1980 revealed unexpected narrow absorption lines in the spectra of HR 6819, and a study of 2003 showed that these lines moved over time. This indicated that although we could not resolve them optically, there were two components of HR 6819: a Be star showing no obvious motion, and a B3 III star in a 40-day orbit.

But what was the star B3 III orbiting? In May 2020, scientists announced an answer to the riddle: HR 6819 must actually be a triple system. The star B3 III, they argued, is orbiting a stellar-mass black hole (which is why we see no evidence for it in the spectra), and the star Be is a distant tertiary companion, orbiting too slowly to have detectable motion.

According to the orbit of the star B3 III, the black hole would need to weigh more than 4 solar masses, and only 1,120 light years distant from Earth, this object would be the closest known black hole. But could there be another explanation for the spectra of HR 6819?

In a new study, published in The Astrophysical Journal Letters, Georgia State University scientists Douglas Gies and Luqian Wang argue that HR 6819 is not a triple system after all. Instead, it is a simple binary, consisting of only the two known components: the star Be and the star B3 III.

If HR 6819 is simply a binary, then the star B3 III should show reflex orbital motion with the same 40-day period, but this motion could be small and difficult to detect in the complex spectra of the system, reports the American Astronomical Society in a statement.

To find it, Gies and Wang analyzed the emission of H-alpha – one of the emission lines in the hydrogen spectrum – from the accretion disk surrounding the star Be. Using a careful spectral model, they show that the entire disk can be seen to move back and forth in a 40-day period, exactly as expected for reflex orbital motion. This movement is about an order of magnitude smaller than that of the star B3 III, which is why it was not previously detected.

So why is the orbital motion of the star Be so much smaller than that of its companion B3 III? If the star Be has a typical mass of about 6 solar masses, the companion must be only a fraction of a solar mass in size. You may be in a stage of evolution where you have already donated a significant amount of mass to your partner, and now only the stripped remnant remains.


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