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

Y. Naze, G. Rauw, and J. Manfroid

Institute of Astrophysics & Geophysics, University of Liege, Belgium

Context: Westerlund 2 is a young, massive, obscured stellar cluster of our Galaxy. It harbors the most massive star with well determined parameters, WR20a (82+83M_sol), a dozen very early O-type stars (O3-7), and a wealth of PMS stars. Although of importance, this cluster is still not well known.
Aims: The high-energy properties of this cluster, especially those of its early-type stars are examined in details. 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 very luminous and display very hard spectra, but WR20b does not seem to vary. On the contrary, WR20a, a known eclipsing, colliding-wind binary, brightens in the X-ray domain during the 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 (2 x 10^24 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 them could be analyzed spectroscopically. Four are overluminous, but the others present typical L_X/L_BOL ratios, suggesting that several O-type objects are actually binaries. Variability at the ~2sigma level was detected for a majority of the sources, of unknown origin for the putatively 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 be at the origin of the TeV source HESS J1023-575.
Finally, the X-ray properties of PMS objects were also investigated, in particular the brightest flaring ones. They provided an additional argument in favor of a large distance (~8kpc) for the cluster.

Reference: Astronomy & Astrophysics
Status: Manuscript has been accepted


Comments: Images of the cluster will soon be added on the Chandra public webpage