SCYON Abstract

Received on October 6 2017

The Structure of the Young Star Cluster NGC 6231. I. Stellar Population

AuthorsM. A. Kuhn (1,2), N. Medina (1,2), K. V. Getman (3), and 4 co-authors
Affiliation(1) Millennium Institute of Astrophysics
(2) Universidad de Valparaiso
(3) Pennsylvania State University
Accepted byAstronomical Journal


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 (assuming a universal initial mass function) with a completeness limit at 0.5 M. 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 catalog members 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 background field stars.