2022MNRAS.510.6133B

We present spectroscopic analysis of the luminous X-ray source Melnick 33Na (Mk 33Na, HSH95 16) in the Large Magellanic Cloud (LMC) 30 Doradus region (Tarantula Nebula), utilizing new time-series Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph spectroscopy. We confirm Mk 33Na as a double-lined O-type spectroscopic binary with a mass ratio q = 0.63 ± 0.02, e = 0.33 ± 0.01, and orbital period of 18.3 ± 0.1 d, supporting the favoured period from X-ray observations obtained via the Tarantula - Revealed by X-rays survey. Disentangled spectra of each component provide spectral types of OC2.5 If* and O4 V for the primary and secondary, respectively. Unusually for an O supergiant the primary exhibits strong C IV 4658 emission and weak N V 4603-20, justifying the OC classification. Spectroscopic analysis favours extreme physical properties for the primary (T_eff = 50 kK, log L/L_☉ = 6.15) with system components of M₁ = 83 ± 19 M_☉ and M₂ = 48 ± 11 M_☉ obtained from evolutionary models, which can be reconciled with results from our orbital analysis (e.g. M₁sin ³i = 20.0 ± 1.2 M_☉) if the system inclination is ∼38° and it has an age of 0.9-1.6 Myr. This establishes Mk 33Na as one of the highest mass binary systems in the LMC, alongside other X-ray luminous early-type binaries Mk34 (WN5h+WN5h), R144 (WN5/6h+WN6/7h), and especially R139 (O6.5 Iafc + O6 Iaf).