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/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 modeling of group-stacked X-ray source spectra .

Results : We find a large cluster population down to \sim 0.3 M _ { \odot } ( complete to \sim 1 M _ { \odot } ) , with minimal non-member contamination , with a definite age spread ( 1-8 Myrs ) 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 \sim 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 \sim 2 dex in mass , has a slope \Gamma \sim - 1.14 .
The total mass of cluster members above 1 M _ { \odot } is 2.28 \times 10 ^ { 3 } M _ { \odot } , and the inferred total mass is 4.38 \times 10 ^ { 3 } M _ { \odot } .
We also study the peculiar , hard X-ray spectrum of the Wolf-Rayet star WR 79 .

Conclusions :