The spectrum of the very massive binary system WR 20a (WN6ha + WN6ha): Fundamental parameters and wind interactions

We analyse the optical spectrum of the very massive binary system WR 20a (WN6ha + WN6ha). The most prominent emission lines, Halpha and He ii lambda 4686, display strong phase-locked profile variability. From the variations of their equivalent widths and from a tomographic analysis, we find that part of the line emission probably arises in a wind interaction region between the stars. Our analysis of the optical spectrum of WR 20a indicates a reddening of A_V ≃ 6.0 mag and a distance of 7.9 kpc, suggesting that the star actually belongs to the open cluster Westerlund 2. The location of the system at ~ 1.1 pc from the cluster core could indicate that WR 20a was gently ejected from the core via dynamical interactions. Using a non-LTE model atmosphere code, we derive the fundamental parameters of each component: T_eff = 43 000 ± 2000 K, log{L_bol/L_⊙} ≃ 6.0, dot{M} = 8.5 ¿ 10^-6 M_⊙ yr^-1 (assuming a clumped wind with a volume filling factor f = 0.1). Nitrogen is enhanced in the atmospheres of the components of WR 20a, while carbon is definitely depleted. Finally, the position of the binary components in the Hertzsprung-Russell diagram suggests that they are core hydrogen burning stars in a pre-LBV stage and their current atmospheric chemical composition probably results from rotational mixing that might be enhanced in a close binary compared to a single star of same age.