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

G. Rauw$^1$, J. Manfroid$^1$, E. Gosset$^1$, Y. Naz'e$^1$, H. Sana$^{2,1}$, M. De Becker$^1$, C. Foellmi$^{3,2}$, A.F.J. Moffat$^4$

1 - Institut d'Astrophysique et de G'eophysique, Universit'e de Li`ege, All'ee du 6 Ao^ut, B^at B5c, 4000 Li`ege, Belgium
2 - European Southern Observatory, Alonso de Cordova 3107, 19 Vitacura, Santiago, Chile
3 - Observatoire de Grenoble, 414, Rue de la Piscine, BP 53, 38041 Grenoble, France
4 - D'epartement de Physique, Universit'e de Montr'eal, QC, H3C 3J7, and Observatoire du Mont M'egantic, Canada

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 RCW49. 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 $8.0 pm 1.4$ 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 RCW49 complex. This suggests that there might still exist a number of embedded early O-stars in RCW49.

Reference: astro-ph/0612622
Status: Manuscript has been accepted

Weblink: http://arxiv.org/abs/astro-ph/0612622


Email: rauw@astro.ulg.ac.be