The Distance to the Massive Galactic Cluster Westerlund 2 from a
Spectroscopic and HST Photometric Study
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by
Carlos A. Vargas Alvarez, Henry A. Kobulnicky, David R. Bradley,
Sheila J. Kannappan, Mark A. Norris, Richard J. Cool, Brendan P. Miller
2013
Abstract
We present a spectroscopic and photometric determination of the distance to
the young Galactic open cluster Westerlund 2 using WFPC2 imaging from the
Hubble Space Telescope and ground-based optical spectroscopy. HST imaging in
the F336W, F439W, F555W, and F814W filters resolved many sources previously
undetected in ground-based observations and yielded photometry for 1136 stars.
We identified fifteen new O-type stars, along with two probable binary systems,
including MSP 188 (O3 + O5.5). We fit reddened SEDs based on the Padova
isochrones to the photometric data to determine individual reddening parameters
R_V and A_V for O-type stars in Wd2. We find average values 〈
R_V〉 = 3.77 ± 0.09 and 〈 A_V〉 = 6.51 ± 0.38 mag,
which result in a smaller distance than most other spectroscopic and
photometric studies. After a statistical distance correction accounting for
close unresolved binaries (factor of 1.08), our spectroscopic and photometric
data on 29 O-type stars yield that Westerlund 2 has a distance 〈 d
〉 = 4.16 ± 0.07 (random) +0.26 (systematic) kpc. The cluster's age
remains poorly constrained, with an upper limit of 3 Myr. Finally, we report
evidence of a faint mid-IR PAH ring surrounding the well-known binary candidate
MSP 18, which appears to lie at the center of a secondary stellar grouping
within Westerlund 2.
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