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

Abstract

Massive O-type stars ($M\gtrsim 15M_{\odot }$) with an X-ray quiet black hole (BH) companion represent a crucial stage in 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. Here we analyse 30 epochs of VLT/FLAMES IFU high-resolution observations of the H$\delta$ region, as well as archival FLAMES spectroscopy, of VFTS 812, a 17-day single-lined spectroscopic binary with an O4V primary and a minimum secondary mass of $5.1M_{\odot }$. Following careful removal of the nebular contamination, spectral disentangling on the new data did not reveal any signature of the hidden companion. We derive $T_{\mathrm{eff}}=49_{-4}^{+3}$ kK, $\log L/L_{\odot }=5.7\pm 0.1$ and $v_{\mathrm{rot},\max }\sin i=110_{-35}^{+25}$ km/s for the O4V component, yielding a (single star) evolutionary mass of $53_{-5}^{+6}$ $M_{\odot }$ and an age in the range of 0-1.6 Myr. Using injection tests of various luminous artificial companions in our data, we exhaustively rule out the presence of any luminous signature from a main sequence star more massive than $6M_{\odot }$. We discuss the possible nature of the companion, suggesting that the 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 robust confirmation and to search for accretion signatures on the O4V star.

8 pages, 6 figures. Accepted in A&A

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