A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function

Context. NGC 6231 is a massive young star cluster, near the center of the Sco OB1 association. While its OB members are well studied, its low-mass population has received little attention. We present high-spatial resolution Chandra ACIS-I X-ray data, where we detect 1613 point X-ray sources. Aims. Our main aim is to clarify global properties of NGC 6231 down to low masses through a detailed membership assessment, and to study the cluster stars' spatial distribution, the origin of their X-ray emission, the cluster age and formation history, and initial mass function. Methods. We use X-ray data, complemented by optical and IR data, to establish cluster membership. The spatial distribution of different stellar subgroups also provides highly significant constraints on cluster membership, as does the distribution of X-ray hardness. We perform spectral modelling of group-stacked X-ray source spectra. Results. We find a large cluster population down to similar to 0.3 M-circle dot (complete to similar to 1 M-circle dot), with minimal non-member contamination, with a definite age spread (1-8 Myr) for the low-mass PMS stars. We argue that low-mass cluster stars also constitute the majority of the few hundreds unidentified X-ray sources. We find mass segregation for the most massive stars. The fraction of circumstellar-disk bearing members is found to be similar to 5%. Photoevaporation of disks under the action of massive stars is suggested by the spatial distribution of the IR-excess stars. We also find strong H alpha emission in 9% of cluster PMS stars. The dependence of X-ray properties on mass, stellar structure, and age agrees with extrapolations based on other young clusters. The cluster initial mass function, computed over similar to 2 dex in mass, has a slope Gamma similar to -1.14. The total mass of cluster members above 1 M-circle dot is 2.28 x 10(3) M-circle dot, and the inferred total mass is 4.38 x 10(3) M-circle dot. We also study the peculiar, hard X-ray spectrum of the Wolf-Rayet star WR 79.