Melnick 33Na: a very massive colliding wind binary system in 30 Doradus

We present spectroscopic analysis of the luminous X-ray source Melnick 33Na (Mk 33Na, HSH95 16) in the LMC 30 Doradus region (Tarantula Nebula), utilising new time-series VLT/UVES spectroscopy. We confirm Mk 33Na as a double-lined O-type spectroscopic binary with a mass ratio $q = 0.63 \pm 0.02$, $e = 0.33 \pm 0.01$ and orbital period of $18.3 \pm 0.1$ days, supporting the favoured period from X-ray observations obtained via the Tarantula -- Revealed by X-rays (T-ReX) 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 CIV 4658 emission and weak NV 4603-20, justifying the OC classification. Spectroscopic analysis favours extreme physical properties for the primary ($T_{\rm eff} = 50$ kK, $\log L/L_{\odot} = 6.15$) with system components of $M_{1} = 83 \pm 19 M_{\odot}$ and $M_{2} = 48 \pm 11 M_{\odot}$ obtained from evolutionary models, which can be reconciled with results from our orbital analysis (e.g. $M_{1} \sin^3 i = 20.0 \pm 1.2 M_{\odot}$) if the system inclination is $\sim 38^{\circ}$ and it has an age of 0.9 to 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).