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

Michael A. Kuhn$^{1,2}$, Nicol\'as Medina$^{1,2}$, Konstantin V. Getman$^{3}$, Eric D. Feigelson$^{3,1}$, Mariusz Gromadzki$^{4,1,2}$, Jordanka Borissova$^{1,2}$, Radostin Kurtev$^{1,2}$

1 - Millennium Institute of Astrophysics, Vicu\~na Mackenna 4860, 7820436 Macul, Santiago, Chile; 2 - Instituto de Fisica y Astronom\'{i}a, Universidad de Valpara\'{i}so, Gran Breta\~{n}a 1111, Playa Ancha, Valpara\'{i}so, Chile; 3 - Department of Astronomy \& Astrophysics, 525 Davey Laboratory, Pennsylvania State University, University Park, PA 16802, USA; 4 - Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland

NGC~6231 is a young cluster (age $\sim$2--7~Myr) dominating the Sco~OB1 association (distance $\sim$1.59~kpc) with $\sim$100 O and B stars and a large pre--main-sequence stellar population. We combine a reanalysis of archival {\it Chandra} X-ray data with multi-epoch NIR photometry from the VVV survey and published optical catalogs to obtain a catalog of 2148 probable cluster members. This catalog is 70\% larger than previous censuses of probable cluster members in NGC~6231, and it includes many low-mass stars detected in the NIR but not in the optical and some B-stars without previously noted X-ray counterparts. In addition, we identify 295 NIR variables, about half of which are expected to be pre--main-sequence stars. With the more-complete sample, we estimate a total population in the {\it Chandra} field of 5700--7500 cluster members down to 0.08~$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 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.

Reference: AJ (in press)
Status: Manuscript has been accepted


Comments: Electronic tables 1, 3, and 4 are available from