TY - JOUR
T1 - The Structure of the Young Star Cluster NGC 6231. I. Stellar Population
AU - Kuhn, Michael A.
AU - Medina, Nicolás
AU - Getman, Konstantin V.
AU - Feigelson, Eric D.
AU - Gromadzki, Mariusz
AU - Borissova, Jordanka
AU - Kurtev, Radostin
N1 - Funding Information:
M.A.K., N.M., E.D.F., M.G., J.B., and R.K. acknowledge support from the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to the Millennium Institute of Astrophysics. M.A.K. was also supported by a fellowship (FONDECYT Proyecto No. 3150319) from the Chilean Comisión Nacional de Investigación Científica y Tecnológica, and R.K. received support from FONDECYT Proyecto No. 1130140. The scientific results reported in this article are based on data obtained from the Chandra Data Archive, the Vista Variables in Vía Lactéa project, and the Two Micron All Sky Survey catalog. This work makes use of analysis methods developed at Penn State for the MYStIX project and Chandra data reduction procedures (including ACIS Extract) developed by Patrick Broos and Leisa Townsley. We thank the referee for many useful comments and suggestions. We also thank A. Sengupta for useful feedback on the article and P. Lucas for expert advice about the VVV survey. Facilities: CXO(ACIS-I), ESO:VISTA(VVV).
Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved.
PY - 2017/9
Y1 - 2017/9
N2 - 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 multiepoch near-infrared (NIR) photometry from the VISTA Variables in the Via Lactéa (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. 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 (assuming a universal initial mass function) with a completeness limit at 0.5 . 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. The 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 companion (>35% of B stars). A small fraction of catalog members have unusually high X-ray median energies or reddened NIR colors, which might be explained by absorption from thick or edge-on disks or being background field stars.
AB - 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 multiepoch near-infrared (NIR) photometry from the VISTA Variables in the Via Lactéa (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. 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 (assuming a universal initial mass function) with a completeness limit at 0.5 . 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. The 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 companion (>35% of B stars). A small fraction of catalog members have unusually high X-ray median energies or reddened NIR colors, which might be explained by absorption from thick or edge-on disks or being background field stars.
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U2 - 10.3847/1538-3881/aa76e8
DO - 10.3847/1538-3881/aa76e8
M3 - Article
AN - SCOPUS:85029123345
VL - 154
JO - Astronomical Journal
JF - Astronomical Journal
SN - 0004-6256
IS - 3
M1 - 87
ER -