Aims:The properties of the early-type stars in the core of the
Methods: 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.
Results: The first light curves of the eclipsing binary WR 20a 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-σ level. The twelve brightest O-type stars are found to have spectral types between O3 and O6.5, significantly earlier than previously thought.
Conclusions: .The distance of the early-type stars in Westerlund 2 is established to be in excellent agreement with the distance of WR 20a, indicating that WR 20a 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.
Astronomy and Astrophysics
- Pub Date:
- March 2007
- Galaxy: open clusters and associations: individual: Westerlund 2;
- stars: early-type;
- stars: binaries: eclipsing;
- stars: fundamental parameters;
- stars: individual: WR 20a;
- 11 pages, 8 figures (figs 1, 3 and 5 in jpg), accepted for publication by A&