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Commission G2 Massive Stars
The dedicated website for the Commission G2 is available here.
Press release
"Closest black hole" system found to contain no black hole
Frost, Bodensteiner et al.
Below is an artist’s impression of a much-discussed exotic stellar system, HR 6819. HR 6819 (d=350pc, V=5.4mag) made headlines in 2020 when it was claimed to be a triple system containing "The nearest black hole to Earth" (Rivinius et al. 2020, A&A, 637, 3). The system showed a complex spectra, with absorption lines associated with a B-type star and emission lines coming from a classical Be star - a B-type star that is rotating so fast it has formed a decretion disc about its equator that is producing strong (particular Balmer) emission. By using multi-epoch FEROS spectra Rivinius et al. thought that the Be star was barely moving and therefore on a very long orbit, whilst the B star was on a 40-day orbit with an unseen object. Others had other ideas though and, for example, Bodensteiner et al. 2020 (A&A, 641, A43, 16) instead found that they could model the spectra with a binary system. In their scenario, the Be star was in the 40-day orbit with the B star, and the B star had to be stripped. This stripping would be the result of an interaction between the two stars, where the B star had expanded during it's main-sequence evolution, went through Roche Lobe overflow, and the subsequent transfer of mass and momentum turned the companion into a fast-rotating B star and stripped the donor star of its envelope. Frost et al. 2022 (A&A. 659, L3, 12) used new observations taken with the MUSE and GRAVITY instruments at the Very Large Telescope/Interferometer to determine which of the two scenarios was the case. With MUSE, they found that there was no Be star at large scales, as required by the black-hole scenario. With GRAVITY they detected the presence of two stars separated by ~1mas, comparable to a 40 day orbit, and in addition found evidence of a stellar-radii sized disc around the brighter star thanks to a double-peaked profile in the spectrum of their GRAVITY data. All this combined implied that HR 6819 was indeed a post-interaction Be+stripped B star binary system, with no black hole.
For more information, see the ESO press release: https://www.eso.org/public/news/eso2204/