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

Abstract

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.

Accepted for publication in The Astronomical Journal; 40 pages, 5 tables, and 17 figures; full versions of Tables 1, 3, and 4 are available at http://astro.psu.edu/mystix

References in corpus (31)

• The distance to the Orion Nebula
• A Spitzer Survey of Young Stellar Clusters within One Kiloparsec of the Sun: Cluster Core Extraction and Basic Structural Analysis
• Evolution of low-mass star and brown dwarf eclipsing binaries
• The XMM-Newton Extended Survey of the Taurus Molecular Cloud (XEST)
• The VST Photometric Halpha Survey of the Southern Galactic Plane and Bulge (VPHAS+)
• An XMM-Newton view of the young open cluster NGC 6231 -- II. The OB star population
• The Spatial Structure of Young Stellar Clusters. I. Subclusters
• X-ray Emission from T Tauri Stars and the Role of Accretion: Inferences from the XMM-Newton Extended Survey of the Taurus Molecular Cloud
• The young stellar population in M17 revealed by Chandra
• X-ray flares in Orion young stars. I. Flare characteristics
• Solar system genealogy revealed by extinct short-lived radionuclides in meteorites
• X-ray Study of Triggered Star Formation and Protostars in IC 1396N
• A Chandra Study of the Rosette Star-Forming Complex. I. The Stellar Population and Structure of the Young Open Cluster NGC 2244
• Young Stellar Object Variability (YSOVAR): Long Timescale Variations in the Mid-Infrared
• The Massive Star-forming Regions Omnibus X-ray Catalog
• Age Gradients in the Stellar Populations of Massive Star Forming Regions Based on a New Stellar Chronometer
• The Diverse Stellar Populations of the W3 Star Forming Complex
• No evidence for mass segregation in massive young clusters
• An X-ray Census of Young Stars in the Massive Southern Star-Forming Complex NGC 6357
• Core-Halo Age Gradients and Star Formation in the Orion Nebula and NGC~2024 Young Stellar Clusters
• The Spatial Structure of Young Stellar Clusters. II. Total Young Stellar Populations
• Protoplanetary Disk Evolution around the Triggered Star Forming Region Cepheus B
• Chandra spectroscopy of the hot star beta Crucis and the discovery of a pre-main-sequence companion
• An XMM-Newton view of the young open cluster NGC 6231 III. Optically faint X-ray sources
• An XMM-Newton view of the young open cluster NGC 6231 -- I. The catalogue
• The Structural Evolution of Forming and Early Stage Star Clusters
• A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function
• The Stellar Population And Origin Of The Mysterious High-Latitude Star Forming Cloud CG 12
• Young Stellar Populations in MYStIX Star Forming Regions: Candidate Protostars
• Establishing the nature of companion candidates to X-ray emitting late B-type stars
• The highly ionized, high velocity gas in NGC 6231

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