A&A 596, A82 (2016) Astronomy DOI: 10.1051/0004-6361/201629029 & c ESO 2016 Astrophysics A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function? F. Damiani, G. Micela, and S. Sciortino INAF−Osservatorio Astronomico di Palermo G. S. Vaiana, Piazza del Parlamento 1, 90134 Palermo, Italy e-mail:
[email protected] Received 31 May 2016 / Accepted 26 July 2016 ABSTRACT 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 dif- ferent 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 ∼0.3 M (complete to ∼1 M ), 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.