The rich low-mass population of the massivecluster NGC 6231: X-ray emission, evolutionarystatus, and IMF

摘要

We present a deep Chandra observation of the young, massive cluster NGC 6231 in ScoOB 1. We detect more than 1600 point X-ray sources, and we argue that about 90% of them arecluster members, thus enlarging by almost a factor of 3 the known total cluster population. Thecolor-magnitude diagram of almost 1100 optical/NIR counterparts shows that the X-ray selectedsample ranges down to 0.3 solar masses, We find a definite age spread (2-10 Myrs) for the low-massstars. Still lower-mass cluster stars will be probably found among the many hundreds unidentifiedX-ray sources, which are themselves clustered. Mass segregation is found for the most massive stars.The fraction of circumstellar disk-bearing members is found to be low (<4%), which might be dueto either spontaneous evolution or photoevaporation under the action of massive stars. A group ofobjects shows particularly hard X-ray spectra. The dependence of X-ray properties on mass, stellarstructure, disk frequency, and cluster evolutionary status agrees with extrapolations drawn on thebasis of the study of younger clusters. We compute the cluster initial mass function over 2.5 ordersof magnitudes in mass, and find that it is well modeled by a Salpeter-like power law, over its wholerange. A previously claimed deficit of low-mass stars is ruled out with high confidence.