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To be published in:
Astronomy & Astrophysics

The spectrum of the very massive binary system WR20a (WN6ha + WN6ha):
fundamental parameters and wind interactions +

G. Rauw1,*, P.A. Crowther2, M. De Becker1, E. Gosset1,*, Y. Nazé1, H. Sana1,**, K.A. van der Hucht3,4, J.-M. Vreux1 and P.M. Williams5

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).


     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 AV ~ 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: Teff = 43000 ± 2000K, log(Lbol/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.

Key Words
binaries: spectroscopic -- stars: early-type -- stars: fundamental parameters -- stars: individual: WR 20a

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