2006/09/11 by Junfeng Wang, Leisa K. Townsley, Eric D. Feigelson +4 · 1 citation
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Cluster (spacecraft) #Initial mass function #Luminosity function #Orion Nebula #Photometry (optics) #Star cluster #Star formation #Stars #Stellar population #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/509147
64 pages (double columns), 9 table, 17 figures (reduced resolution), ApJ accepted. Please contact J. Wang for full tables
arxiv created 2006/09/11 · openalex publication_date 2007/01/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
We present the first high spatial resolution X-ray study of the massive star-forming region NGC 6357, obtained in a 38 ks Chandra /ACIS observation. Inside the brightest constituent of this large H II region complex is the massive open cluster Pismis 24. It contains two of the brightest and bluest stars known, yet remains poorly studied; only a handful of optically bright stellar members have been identified. We investigate the cluster extent and initial mass function and detect ~800 X-ray sources with a limiting sensitivity of ~10 30 ergs s -1 ; this provides the first reliable probe of the rich intermediate-mass and low-mass population of this massive cluster, increasing the number of known members from optical study by a factor of ~50. The high-luminosity end (log L h [2-8 keV] ≥ 30.3 ergs s -1 ) of the observed X-ray luminosity function in NGC 6357 is clearly consistent with a power-law relation as seen in the Orion Nebula Cluster and Cepheus B, yielding the first estimate of NGC 6357's total cluster population, a few times the known Orion population. We investigate the structure of the cluster, finding small-scale substructures superposed on a spherical cluster with 6 pc extent, and discuss its relationship to the nebular morphology. The long-standing L X - 10 -7 L bol correlation for O stars is confirmed. Twenty-four candidate O stars and one possible new obscured massive YSO or Wolf-Rayet star are presented. Many cluster members are estimated to be intermediate-mass stars from available infrared photometry (assuming an age of ~1 Myr), but only a few exhibit K -band excess. We report the first detection of X-ray emission from an evaporating gaseous globule at the tip of a molecular pillar; this source is likely a B0-B2 protostar.