2005A&A...432..985R

We analyse the optical spectrum of the very massive binary system WR20a (WN6ha + WN6ha). The most prominent emission lines, Hα and Heii λ 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 WR20a indicates a reddening of A_V≃6.0mag and a distance of ∼7.9kpc, suggesting that the star actually belongs to the open cluster Westerlund2. The location of the system at ∼1.1pc from the cluster core could indicate that WR20a 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=43000±2000K, logL_bol/L_☉≃6.0, {dot}(M)=8.5x10^–6M_☉/yr (assuming a clumped wind with a volume filling factor f=0.1). Nitrogen is enhanced in the atmospheres of the components of WR20a, 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.