P99	December 2004

¹ Institut d'Astrophysique et de Géophysique - Université de Liège, Allée du 6 Août, Bât B5c, B-4000 Liège (Sart Tilman), Belgium
² Department of Physics & Astronomy, University of Sheffield, Hicks Building, Hounsfield Rd., Sheffield, S3 7RH, UK
³ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁴ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁵ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK

^* Research Associate FNRS, Belgium
^** Research Fellow FNRS, Belgium

⁺ Based on observations collected at the European Southern Observatory (La Silla, Chile).

Abstract

     We analyse the optical spectrum of the very massive binary system WR20a (WN6ha + WN6ha). The most prominent emission lines, H and He II 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 ~ 6.0 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 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, log(L_bol/L) ~ 6.0, = 8.5 10^-6 Myr^-1 (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.