The distance to the young open cluster Westerlund 2

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 + 0.02 E 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 c JHK 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 × 10 6 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.

Spatial distribution of Galactic Wolf–Rayet stars and implications for the global population

“…However, these distances are derived on the asumption of an RV = 3.1 extinction law, yielding an average AV = 4.68 for the cluster. Carraro et al (2012) have claimed an anomalous extinction law along this line of sight with an average RV = 3.8± 0.2. These authors use the spectra of Rauw et al to obtain AV = 7.1± 1.2, corresponding to a much smaller distance of 3.02 ± 0.52 kpc.…”

A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope. Ii. Mass Accretion in the Pre-Main-Sequence Population

“…There is general agreement in the literature that Wd2 is younger than 3 Myr and that its core might be younger than 2 Myr (Ascenso et al 2007;Carraro et al 2013). In our first paper (Zeidler et al 2015, hereafter Paper I) we confirmed the cluster distance of VargasÁlvarez et al (2013) of 4.16 kpc, using Hubble Space Telescope (HST) photometry and our highresolution 2D extinction map.…”

On the crucial cluster Andrews-Lindsay 1 and a 4% distance solution for its planetary nebula

“…That analysis relies on WISE midinfrared data (Wright et al 2010) to determine extinction law variations via the color-extrapolation method. For comparison, certain regions in Carina and the Galactic bulge adhere to an anomalous extinction law (Pietrukowicz et al 2012;Carraro et al 2013).…”