A few months ago, a group of scientists announced the indirect detection of the closest black hole to Earth in the star system HR 6819 and stated that it is a triple system made up of a star Be and a B3 III, which orbits said black hole in 40 days. However, this week, Georgia State University experts Douglas Gies and Luqian Wang have reached a conclusion that breaks their schemes and determined that it does not have that structure.
From the earliest spectra of the HR 6819 star system, scientists identified this source as a bright, early-type Be star: a hot star with emission lines, probably due to the accumulation of a disk of circumstellar material.
However, studies conducted in the 1980s revealed unexpected narrow absorption lines in the absorption spectra, and a 2003 study showed that these lines moved over time. This indicated that although it was not possible to 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.
In May 2020, experts 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 and the star Be is a distant tertiary companion, orbiting too slowly to have detectable motion. A black hole that would need to weigh more than 4 solar masses and only 1,120 light years distant from Earth, making it the closest to it.
A new study, published in The Astrophysical Journal Letters, argues that HR 6819 is not a triple system, after all. Instead, is a simple binary, which consists 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 a reflex orbital motion with the same 40-day period, but this could be small and difficult to detect in the complex spectra of the system.
Thus, through careful spectral modeling, they have shown that it can be seen that the whole disk moves forward and back over a 40-day period, exactly as expected for reflex orbital motion. This is about an order of magnitude smaller than that of the star B3 III, which is why it was not previously detected.