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

We present deep UBVIC photometric data of the young open cluster Westerlund 2. An abnormal reddening law of RV, cl = 4.14 ± 0.08 was found for the highly reddened early-type members (E(B−V)≧1.45E(B−V)≧1.45), whereas a fairly normal reddening law of RV, fg = 3.33 ± 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 fitting to the reddening-corrected colour–magnitude diagram of the early-type members to be V0 − MV = 13.9 ± 0.14 (random error) +0.4−0.1−0.1+0.4 (the upper limit of systematic error) mag (d=6.0±0.4+1.2−0.3d=6.0±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 (Galactic Legacy Infrared Mid-Plane Survey Extraordinaire) source catalogue. The initial mass function shows a shallow slope of Γ = −1.1 ± 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 ≲ 1.5 Myr. We confirmed the existence of a clump of PMS stars located ∼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