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

Massive O-type stars (M ≳ 15 M_⊙) 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 ≲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 Hδ 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_⊙. Following careful removal of the nebular contamination, spectral disentangling on the new data did not reveal any signature of the hidden companion. We derive T_eff = 49⁺³₋₄ T eff = 49 − 4 + 3 kK, log L = 5.7 ± 0.1, and v_rot,max sin i = 110⁺²⁵₋₃₅ v rot , max sin i = 110 − 35 + 25 km s⁻¹ for the O4 V component, yielding a (single-star) evolutionary mass of 53 = ⁺⁶₋₅ 53 − 5 + 6 M ⊙ 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_⊙. 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.