Weighing Melnick 34: the most massive binary system known

Here, we confirm Melnick 34, an X-ray bright star in the 30 Dor region of the Large Magellanic Cloud, as an SB2 binary comprising WN5h + WN5h components. We present orbital solutions using 26 epochs of VLT/UVES spectra and 22 epochs of archival Gemini/GMOS spectra. Radial velocity monitoring and automated template-fitting methods both reveal a similar high-eccentricity system with a mass ratio close to unity, and an orbital period in agreement with the 155.1 ± 1 d X-ray light-curve period previously derived by Pollock et al. Our favoured solution derived an eccentricity of 0.68 ± 0.02 and mass ratio of 0.92 ± 0.07, giving minimum masses of M_Asin³(i) = 65 ± 7 M_☉ and M_Bsin³(i) = 60 ± 7 M_☉. Spectral modelling using WN5h templates with CMFGEN reveals temperatures of T ̃ 53 kK for each component and luminosities of log(L_A/L_☉) = 6.43 ± 0.08 and log(L_B/L_☉) = 6.37 ± 0.08, from which BONNSAI evolutionary modelling gives masses of M_A = 139^{+21}_{-18} M_☉ and M_B = 127^{+17}_{-17} M_☉ and ages of ̃0.6 Myr. Spectroscopic and dynamic masses would agree if Mk34 has an inclination of i ̃ 50°, making Mk34 the most massive binary known and an excellent candidate for investigating the properties of colliding wind binaries. Within 2-3 Myr, both components of Mk34 are expected to evolve to stellar mass black holes, which, assuming the binary system survives, would make Mk34 a potential binary black hole merger progenitor and a gravitational wave source.