A DEEP CHANDRA X-RAY OBSERVATION OF THE RICH YOUNG CLUSTER NGC 6530. Ⅰ. THE X-RAY SOURCE CATALOG AND THE CLUSTER POPULATION

摘要

In a deep 60 ks Chandra ACIS X-ray observation of the very young cluster NGC 6530, we detect 884 X-ray point sources and argue that a very large fraction of them (90%-95%) must be pre-main-sequence (PMS) cluster members, mostly of low masses. This is a significant enlargement of the known NGC 6530 stellar population with respect to previous optical studies, including Hα surveys. We identify 220 X-ray sources with cataloged stars down to V = 17, while most unidentified sources have fainter counterparts. Moreover, we find an infrared counterpart in the 2MASS catalog for 731 X-ray sources. The optically identified cluster X-ray sources are found in a band in the H-R diagram above the main sequence, in the locus of 0.5-1.5 Myr PMS stars, with masses down to 0.5-1.5 solar mass. We find evidence of an age gradient across the field from northwest to south, suggesting a sequence of star formation events qualitatively similar to that found in earlier studies of the same region, but differing in the details. A group of X-ray sources showing frequent flares may be associated with the youngest stars in the cluster, suggesting that X-ray flaring activity is especially intense in the youngest PMS phases of low-mass stars.