Reddening, distance, and stellar content of the young open cluster Westerlund 2

We present deep UBVI sub(C) photometric data of the young open cluster Westerlund 2. An abnormal reddening law of R sub(V, cl) = 4.14 plus or minus 0.08 was found for the highly reddened early-type members (E(B-V) greater than or equal to 1.45), whereas a fairly normal reddening law of R sub(V, fg) = 3.33 plus or minus 0.03 was confirmed for the foreground early-type stars (E(B - V) sub(fg) < 1.05). The distance modulus was determined from zero-age main-sequence fitting to the reddening-corrected colour-magnitude diagram of the early-type members to be V sub(0) - M sub(V) = 13.9 plus or minus 0.14 (random error) super(+0.4) sub(-0.1) (the upper limit of systematic error) mag (d = 6.0 plus or minus 0.4 super(+1.2) sub(-0.3) 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 (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire) source catalogue. The initial mass function shows a shallow slope of Gamma = -1.1 plus or minus 0.1 down to log m = 0.7. The total mass of Westerlund 2 is estimated to be at least 7 400 M. The age of Westerlund 2 from the main-sequence turn-on and PMS stars is estimated to be less than or equal to 1.5 Myr. We confirmed the existence of a clump of PMS stars located similar to 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.