We present deep UBVI _ { 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 ) \gid 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 _ { \sun } .
The age of Westerlund 2 from the main-sequence turn-on and PMS stars is estimated to be \la 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 .