An X-ray imaging study of the stellar population in RCW 49

We present the results of a high-resolution X-ray imaging study of the stellar population in the Galactic massive star-forming region RCW 49 and its central OB association Westerlund 2. We obtained a ∼40 ks X-ray image of a ∼ 17′ × 17′ field using the Chandra X-Ray Observatory and deep NIR images using the Infrared Survey Facility in a concentric ∼ 8.3′ × 8.3′ region. We detected 468 X-ray sources and identified optical, NIR, and Spitzer MIR counterparts for 379 of them. The unprecedented spatial resolution and sensitivity of the X-ray image, enhanced by optical and infrared imaging data, yielded the following results: (1) The central OB association Westerlund 2 is resolved for the first time in the X-ray band. X-ray emission is detected from all spectroscopically identified early-type stars in this region. (2) Most (∼86%) X-ray sources with optical or infrared identifications are cluster members in comparison with a control field in the Galactic plane. (3) A loose constraint (2-5 kpc) for the distance to RCW 49 is derived from the mean X-ray luminosity of T Tauri stars. (4) The cluster X-ray population consists of low-mass pre-main-sequence and early-type stars as obtained from X-ray and NIR photometry. About 30 new OB star candidates are identified. (5) We estimate a cluster radius of 6′-7′ based on the X-ray surface number density profiles. (6) A large fraction (∼90%) of cluster members are identified individually using complimentary X-ray and MIR excess emission. (7) The brightest five X-ray sources, two Wolf-Rayet stars and three O stars, have hard thermal spectra.