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

الأصل · EN

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)≥ 1.45), 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 (ZAMS) fitting to the reddening-corrected colour-magnitude diagram of the early-type members to be V₀-Mᵥ=13.9±0.14 (random error) ₋₀.₁⁺⁰.⁴ (the upper limit of systematic error) mag (d = 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 source catalog. The initial mass function shows a shallow slope of Γ=-1.1 ± 0.1 down to 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.