SCYON Abstract

Received on February 5 2007

Early-type stars in the core of the young open cluster Westerlund 2

AuthorsG. Rauw (1), J. Manfroid (1), E. Gosset (1), Y. Nazé (1), H. Sana (2,1), M. De Becker (1), C. Foellmi (3,2), and A.F.J. Moffat (4)
(1) Institut d'Astrophysique et de Géophysique, Université de Liège
(2) European Southern Observatory, Chile
(3) Observatoire de Grenoble, France
(4) Département de Physique, Université de Montréal, and Observatoire du Mont Mégantic, Canada
Accepted byAstronomy & Astrophysics
Links Westerlund 2


The properties of the early-type stars in the core of the Westerlund 2 cluster are examined in order to establish a link between the cluster and the very massive Wolf-Rayet binary WR20a as well as the H II complex RCW 49. Photometric monitoring as well as spectroscopic observations of Westerlund 2 are used to search for light variability and to establish the spectral types of the early-type stars in the cluster core. The first light curves of the eclipsing binary WR20a in B and V filters are analysed and a distance of 8 kpc is inferred. Three additional eclipsing binaries, which are probable late O or early B-type cluster members, are discovered, but none of the known early O-type stars in the cluster displays significant photometric variability above 1% at the 1-sigma level. The twelve brightest O-type stars are found to have spectral types between O3 and O6.5, significantly earlier than previously thought. The distance of the early-type stars in Westerlund 2 is established to be in excellent agreement with the distance of WR20a, indicating that WR20a actually belongs to the cluster. Our best estimate of the cluster distance thus amounts to 8.0 +/- 1.4 kpc. Despite the earlier spectral types, the currently known population of early-type stars in Westerlund 2 does not provide enough ionizing photons to account for the radio emission of the RCW 49 complex. This suggests that there might still exist a number of embedded early O-stars in RCW 49.