A High-resolution Multiband Survey of Westerlund 2 with the Hubble Space Telescope. I. Is the Massive Star Cluster Double?

We present first results from a high resolution multi-band survey of the Westerlund 2 region with the Hubble Space Telescope. Specifically, we imaged Westerlund 2 with the Advanced Camera for Surveys through the F555W, F814W, and F658N filters and with the Wide Field Camera 3 in the F125W, F160W, and F128N filters. We derive the first high resolution pixel-to-pixel map of the color excess E(B-V)g of the gas associated with the cluster, combining the Hα (F658N) and Paβ (F128N) line observations. We demonstrate that, as expected, the region is affected by significant differential reddening with a median of E(B-Vg=1.87 mag. After separating the populations of cluster members and foreground contaminants using a (F814W-F160W) versus F814W color-magnitude diagram, we identify a pronounced pre-main-sequence population in Westerlund 2 showing a distinct turn-on. After dereddening each star of Westerlund 2 individually in the color-magnitude diagram we find via over-plotting PARSEC isochrones that the distance is in good agreement with the literature value of ~4.16 ± 0.33 kpc. With zero-age main sequence fitting to two-color-diagrams, we derive a value of total to selective extinction of RV=3.95 ± 0.135. A spatial density map of the stellar content reveals that the cluster might be composed of two clumps. We estimate the same age of 0.5-2.0 Myr for both clumps. While the two clumps appear to be coeval, the northern clump shows a ~20\% lower stellar surface density.