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

Michael A. Kuhn, Nicolás Medina, Konstantin V. Getman, Eric D. Feigelson, Mariusz Gromadzki, Jordanka Borissova, Radostin Kurtev

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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 Vía 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 {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. 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.
Original languageEnglish
Article number87
JournalThe Astronomical Journal
Publication statusPublished - 1 Sept 2017


  • open clusters and associations: individual: NGC 6231
  • stars: formation
  • stars: massive
  • stars: pre-main sequence
  • stars: variables: T Tauri
  • Herbig Ae/Be
  • X-rays: stars
  • Astrophysics - Astrophysics of Galaxies
  • Astrophysics - High Energy Astrophysical Phenomena
  • Astrophysics - Solar and Stellar Astrophysics


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