The Structure of the Young Star Cluster NGC 6231. I. Stellar Population
  NGC 6231 is a young cluster (age ~2-7 Myr) dominating the Sco OB1 association
(distance ~1.59 kpc) with ~100 O and B stars and a large pre-main-sequence
stellar population. We combine a reanalysis of archival Chandra X-ray data with
multi-epoch NIR photometry from the VVV survey and published optical catalogs
to obtain a catalog of 2148 probable cluster members. This catalog is 70%
larger than previous censuses of probable cluster members in NGC 6231, and it
includes many low-mass stars detected in the NIR but not in the optical and
some B-stars without previously noted X-ray counterparts. In addition, we
identify 295 NIR variables, about half of which are expected to be
pre-main-sequence stars. With the more-complete sample, we estimate a total
population in the Chandra field of 5700-7500 cluster members down to 0.08
$M_\odot$ (assuming a universal initial mass function) with a completeness
limit at 0.5 $M_\odot$. A decrease in stellar X-ray luminosities is noted
relative to other younger clusters. However, within the cluster, there is
little variation in the distribution of X-ray luminosities for ages less than 5
Myr. X-ray spectral hardness for B stars may be useful for distinguishing
between early-B stars with X-rays generated in stellar winds and B-star systems
with X-rays from a pre-main-sequence companions (>35% of B stars). A small
fraction of pre-main-sequence stars have unusually high X-ray median energies
or reddened near-infrared colors, which might be explained by absorption from
thick or edge-on disks or being a background field star.
