The distance to the young open cluster Westerlund 2^⋆,⋆⋆

¹ ESO, Alonso de Cordova 3107, 19001 Santiago de Chile Chile
e-mail: gcarraro@eso.org
² Department of Astronomy and Physics, Saint Mary’s University, Halifax, NS B3H 3C3, Canada
e-mail: turner@ap.smu.ca, dmajaess@ap.smu.ca
³ Facultad de Ciencias Astronómicas y Geofísicas (UNLP), Instituto de Astrofísica de La Plata (CONICETUNLP), Paseo del Bosque s/n, La Plata, Argentina
e-mail: gbaume@fcaglp.unlp.edu.ar

Received: 6 March 2013
Accepted: 17 May 2013

Abstract

A new X-ray, UBVRI_c, 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 E_U − B/E_B − V = 0.63 + 0.02E_B − V. Variable-extinction analyses for Westerlund 2 and nearby IC 2581 based upon spectroscopic distance moduli and ZAMS fitting yield values of R_V = A_V/E_B − 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 UBVRI_cJHK_s data for 19 spectroscopically observed cluster members, yielding R_V = 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.