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

We present deep UBVIC photometric data of the young open clusterWesterlund 2. An abnormal reddening law of R_{V, cl} = 4.14 ± 0.08 was found for the highly reddened early-type members (E(B - V ) ≧ 1.45), whereas a fairly normal reddening law of R_{V, 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 -M_V =13.9±0.14 (random error) ^+0.4 _-0.1 (the upper limit of systematic error) mag (d = 6.0 ± 0.4^+1.2 _-0.3 kpc). To obtain the initial mass function, premain- 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 logm = 0.7. The total mass of Westerlund 2 is estimated to be at least 7 400M⊙. 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.