AAS 205th Meeting, 9-13 January 2005
Session 22 Open Clusters Young and Old
Poster, Monday, January 10, 2005, 9:20am-6:30pm, Exhibit Hall

## [22.13] Revealing the Nature of Faint X-Ray Sources in the Giant Star-Forming Region NGC 3603

C. Poteet, S. Marchenko (Western Kentucky University), M. Corcoran (USRA, GSFC), M. Andersen (Steward Obs.)

NGC 3603, an open cluster embedded in the largest Galactic {\sc H II} region, contains some of the most luminous, massive stars known in the Galaxy. It may serve as a good analog of star-forming regions in external starburst galaxies. The recent deep (50 Ksec) {Chandra} imaging of NGC 3603 revealed that its X-Ray emission is dominated by the bright Wolf-Rayet and O stars in the cluster core. However, there are hundreds of extremely weak point-like X-ray sources surrounding the central region. They tend to concentrate towards the cluster's center, suggestive of real cluster membership. Studying this population, we find that absolute majority of the weak sources does not belong to the lists of known OB and WR cluster members. Practically all the 263 weak X-ray sources have faint near-IR counterparts in the deep J,H,K images obtained by the ISAAC instrument on the VLT. Around 50% of the X-ray sources can be considered as visual binaries, an additional ~20% as multiple systems (up to six stars in a r=1.5"~3\sigma circle). The composite spectrum of these sources is quite hard (kT~6 keV) and fairly absorbed (NH~1022). There is no obvious line emission, in particular, no strong Fe XXV line, so presently we cannot distinguish between thermal and non-thermal emission. The JHK colour-colour and colour-magnitude diagrams of the X-ray selected sources indicate that the X-ray data are very efficient in discriminating between field stars and cluster members. The X-ray sources have similar near-infrared properties as the whole near-infrared sample of the known cluster members. Hence, the discovered population of weak X-ray sources provide an effective means to obtain a "clean" sample of pre-main sequence stars down to 1 M\odot assuming the cluster is 1 Myr old.

Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.