The young stellar population in M17 revealed by Chandra
  We report here results from a Chandra ACIS observation of the stellar populations in and around the Messier 17 H II region. The field reveals 886 sources, 771 of which have stellar counterparts in infrared images. In addition to comprehensive tables of X-ray source properties, several results are presented: * The X-ray Luminosity Function is calibrated to that of the Orion Nebula Cluster population to infer a total population of roughly 8000--10,000 stars * About 40% of the ACIS sources are heavily obscured with A_V > 10 mag. Some are concentrated around well-studied star-forming regions but most are distributed across the field. X-ray emission is detected from 64 of the hundreds of Class I protostar candidates that can be identified by near- and mid-infrared colors. These constitute the most likely protostar candidates known in M17. * The spatial distribution of X-ray stars is complex: we find a new embedded cluster, a 2 pc-long arc of young stars along the southwest edge of the M17 H II region, and 0.1 pc substructure within various populations. These structures may indicate that the populations are dynamically young. * All (14/14) of the known O stars but only about half (19/34) of the known B0--B3 stars in the M17 field are detected. Six of these stars exhibit very hard thermal plasma components (kT>4 keV) that may be due to colliding wind binaries. More than 100 candidate new OB stars are found, including 28 X-ray detected intermediate- and high-mass protostar candidates with infrared excesses. * Only a small fraction (perhaps 10%) of X-ray selected high- and intermediate- mass stars exhibit K-band emitting protoplanetary disks, providing further evidence that inner disks evolve very rapidly around more massive stars. 
Patrick S. Broos, Eric D. Feigelson, Leisa K. Townsley, Konstantin V.   Getman, Junfeng Wang, Gordon P. Garmire, Zhibo Jiang...