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 MAsin3(i) = 65 ± 7 M⊙ and MBsin3(i) = 60 ± 7 M⊙. Spectral modelling using WN5h templates with cmfgen reveals temperatures of T ∼ 53 kK for each component and luminosities of log(LA/L⊙) = 6.43 ± 0.08 and log(LB/L⊙) = 6.37 ± 0.08, from which BONNSAI evolutionary modelling gives masses of MA = 139+21−18 M⊙ and MB = 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.