Volume 555, July 2013
|Number of page(s)||9|
|Section||Galactic structure, stellar clusters and populations|
|Published online||27 June 2013|
ESO, Alonso de Cordova 3107, 19001
Santiago de Chile
2 Department of Astronomy and Physics, Saint Mary’s University, Halifax, NS B3H 3C3, Canada
e-mail: firstname.lastname@example.org, email@example.com
3 Facultad de Ciencias Astronómicas y Geofísicas (UNLP), Instituto de Astrofísica de La Plata (CONICETUNLP), Paseo del Bosque s/n, La Plata, Argentina
Received: 6 March 2013
Accepted: 17 May 2013
A new X-ray, UBVRIc, and JHKs study of the young cluster Westerlund 2 was undertaken to resolve discrepancies tied to the cluster’s distance. Existing spectroscopic observations for bright cluster members and new multi-band photometry imply a reddening relation toward Westerlund 2 described by EU − B/EB − V = 0.63 + 0.02EB − V. Variable-extinction analyses for Westerlund 2 and nearby IC 2581 based upon spectroscopic distance moduli and ZAMS fitting yield values of RV = AV/EB − V = 3.88 ± 0.18 and 3.77 ± 0.19, respectively, and confirm prior assertions that anomalous interstellar extinction is widespread throughout Carina. The results were confirmed by applying the color-difference method to UBVRIcJHKs data for 19 spectroscopically observed cluster members, yielding RV = 3.85 ± 0.07. The derived distance to Westerlund 2 of d = 2.85 ± 0.43 kpc places the cluster on the far side of the Carina spiral arm. The cluster’s age is no more than τ ~ 2 × 106 yr as inferred from the cluster’s brightest stars and an X-ray (Chandra) cleaned analysis of its pre-main-sequence demographic. Four Wolf-Rayet stars in the cluster core and surrounding corona (WR20a, WR20b, WR20c, and WR20aa) are very likely cluster members, and their inferred luminosities are consistent with those of other late-WN stars in open clusters. The color–magnitude diagram for Westerlund 2 also displays a gap at spectral type B0.5 V with associated color spread at higher and lower absolute magnitudes that might be linked to close binary mergers. These features, in conjunction with the evidence for mass loss from the WR stars, may help to explain the high flux of γ-rays, cosmic rays, and X-rays from the direction toward Westerlund 2.
Key words: stars: Wolf-Rayet / open clusters and associations: general / open clusters and associations: individual: Westerlund 2 / Hii regions / stars: massive
Full photometric data are available at the CDS via anomymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/555/A50
© ESO, 2013
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