The Tarantula massive binary monitoring VII. The nature of the eccentric O+BH binary candidate VFTS 812

Massive O-type stars (\(M \gtrsim 15\,M_\odot\)) with an X-ray-quiet black hole (BH) companion represent a crucial stage in the massive binary evolution leading to binary BH mergers. The population of such binaries remains elusive, with \(\lesssim\) 5 candidate or confirmed systems. The Tarantula nebula harbors thousands of massive stars, 2–3% of which are expected to have BH companions. It is therefore an ideal place to hunt for such systems. We analyzed 30 epochs of VLT/FLAMES IFU high-resolution observations of the H\(\delta\) region and archival FLAMES spectroscopy of VFTS 812, a 17-day single-lined spectroscopic binary with an O4 V primary and a minimum secondary mass of 5.1 \(M_\odot\). Following careful removal of the nebular contamination, spectral disentangling on the new data did not reveal any signature of the hidden companion. We derive Teff = \(49^{+3}_{-4}\) kK, log L = 5.7 \(\pm\) 0.1, and vrot,max sin i = \(110^{+25}_{-35}\,{\rm km s}^{-1}\) for the O4 V component, yielding a (single-star) evolutionary mass of \(53^{+6}_{-5}\,M_\odot\) and an age in the range 0–1.6 Myr. Using injection tests for various luminous artificial companions in our data, we exhaustively ruled out the presence of any luminous signature from a main sequence star more massive than 6 \(M_\odot\). We discuss the possible nature of the companion, suggesting that a rejuvenated O star + BH companion is the most suitable scenario to consistently explain the location, (rejuvenated) young age, eccentricity, and lack of companion signature. While this establishes VFTS 812 as a strong candidate O+BH system, follow-up observations are deemed necessary for a robust confirmation and to search for accretion signatures on the O4 V star.

Recommended citation: Deshmukh et al. (2026), A&A 706, L17
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