Letter to the Editor

The Tarantula massive binary monitoring

VII. The nature of the eccentric O + BH binary candidate VFTS 812^★

¹ Institute of Astronomy, KU Leuven Celestijnlaan 200D 3001 Leuven, Belgium
² Leuven Gravity Institute, KU Leuven Celestijnenlaan 200D box 2415 3001 Leuven, Belgium
³ Sterrenkundig Observatorium, Universiteit Gent Krijgslaan 281 S9 B-9000 Gent, Belgium
⁴ The School of Physics and Astronomy, Tel Aviv University Tel Aviv 6997801, Israel
⁵ Research Center for the Early Universe, Graduate School of Science, The University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113-0033, Japan
⁶ Royal Observatory of Belgium B-1180 Brussels, Belgium
⁷ Max-Planck-Institut für Astronomie Königstuhl 17 D-69117 Heidelberg, Germany

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 January 2026
Accepted: 3 February 2026

Abstract

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 kK, log L = 5.7 ± 0.1, and v_rot,max sin i = 110⁺²⁵₋₃₅ km s⁻¹ for the O4 V component, yielding a (single-star) evolutionary mass of 53 = ⁺⁶₋₅ 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.