Chandra monitoring of the very massive binary WR20a and the young massive cluster Westerlund 2

Institut d'Astrophysique et de Géophysique, Université de Liège, 17 allée du 6 août, Bât. B5C, 4000-Liège, Belgium e-mail: naze@astro.ulg.ac.be

Received: 15 October 2007
Accepted: 23 December 2007

Abstract

Context. Westerlund 2 is a young and massive, obscured stellar cluster of our Galaxy. It harbors the most massive star with well-determined parameters, WR20a (82 + 83 ), a dozen very early O-type stars (O3–7), and a wealth of PMS stars. Although of clear significance, this star cluster has not been well-studied.

Aims. The high-energy properties of this cluster, especially those of its early-type stars are examined in detail. The variability of the X-ray sources is investigated.

Methods. A monitoring of the field was performed using three Chandra observations. This dataset probes daily as well as monthly to yearly timescales and provides the deepest X-ray view of the cluster to date.

Results. The two Wolf-Rayet stars WR20a (WN6ha+WN6ha) and WR20b (WN6ha) were analyzed in detail. They are both luminous and display hard spectra, but WR20b does not appear to vary. In contrast, WR20a, a known eclipsing, colliding-wind binary, brightens in the X-ray domain during eclipses, i.e. when the collision is seen face-on. This can be explained by the properties of the wind-wind collision zone, whose high density leads to a large absorbing column ( cm^-2). All twelve O-type stars previously classified spectroscopically, two eclipsing binaries previously identified and nine newly-identified, O-type star candidates are detected in the high-energy domain; ten of those could be analyzed spectroscopically. Four are overluminous, but the others have typical ratios, suggesting that several O-type objects are actually binaries. Variability at the ~ level is detected for a majority of the sources, of unknown origin for single objects. Faint, soft, diffuse emission pervades the entire field-of-view but no clear structure can be identified, even at the position of a blister proposed to correspond to the TeV source HESS J1023-575. The X-ray properties of PMS objects, in particular the brightest flaring ones, are also investigated. They provided an additional argument in favor of a large distance (~8 kpc) for the cluster.