Reddening, Distance, and Stellar Content of the Young Open Cluster Westerlund 2

Abstract

We present deep $UBV{I}_C$ photometric data of the young open cluster Westerlund 2. An abnormal reddening law of $R_{V,cl}=4.14\pm 0.08$ was found for the highly reddened early-type members ($E(B-V)\ge 1.45$), whereas a fairly normal reddening law of $R_{V,fg}=3.33\pm 0.03$ was confirmed for the foreground early-type stars ($E(B-V{)}_{fg}<1.05$). The distance modulus was determined from zero-age main-sequence (ZAMS) fitting to the reddening-corrected colour-magnitude diagram of the early-type members to be $V_0-M_V=13.9\pm 0.14$ (random error) ${}_{-0.1}^{+0.4}$ (the upper limit of systematic error) mag ($d=6.0\pm 0.4_{-0.3}^{+1.2}$ kpc). To obtain the initial mass function, pre-main-sequence (PMS) stars were selected by identifying the optical counterparts of Chandra X-ray sources and mid-infrared emission stars from the Spitzer GLIMPSE source catalog. The initial mass function shows a shallow slope of $\Gamma =-1.1\pm 0.1$ down to $\log m=0.7$. The total mass of Westerlund 2 is estimated to be at least 7,400 $M_{\odot }$. The age of Westerlund 2 from the main-sequence turn-on and PMS stars is estimated to be $\lesssim$ 1.5 Myr. We confirmed the existence of a clump of PMS stars located $\sim 1$ arcmin north of the core of Westerlund 2, but we could not find any clear evidence for an age difference between the core and the northern clump.

25 pages, 22 figures, 6 tables, accepted for publication in MNRAS

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