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(A)sin(3)(i) = 65 +/- 7 M(circle dot )and M(B)sin(3) (i) = 60 +/- 7 M-circle dot. Spectral modelling using WN5h templates with CMFGEN reveals temperatures of T similar to 53 kK for each component and luminosities of log(L-A/L-circle dot) = 6.43 +/- 0.08 and log(L-B/L-circle dot) = 6.37 +/- 0.08, from which BONNSAI evolutionary modelling gives masses of M-A = 139(-18)(+21) M-circle dot and M-B = 127(-17)(-17) M-circle dot and ages of similar to 0.6 Myr. Spectroscopic and dynamic masses would agree if Mk34 has an inclination of i similar to 50 degrees, 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.