The Reddening Law and Distance of the Highly Reddened Massive Young Open Cluster Westerlund 2

摘要

Westerlund 2 is one of the most reddened massive young open clusters observable in optical bands. There is large disagreement among previous distance determinations. The main cause of the disagreement seems to be due to the abnormal reddening law. We report the reddening law and distance of Westerlund 2 using the deep UBVI_c. photometry data obtained with the CTIO 4m Blanco telescope and MOSAIC II CCD camera. We estimated the reddening E(B - V) Using the relation E(U - B)/E(B - V) = 0.72 + 0.025E(B - V). The foreground reddening law is found to be fairly normal, E(V - I)/E(B - V) = 1.31 (i.e. R_v ≡ A_v/E(B - V) = 3.2) for E(B - V) ≤ 1.05. An abnormal reddening law of Rv = 4.3 is determined for E(B - V) >1.05 stars using the combined optical and published near-IR data. We also report the color excess ratio E(V - I)/E(B - V) for highly reddened OB stars may not follow the relation R_v= 2.45E(V - I)/E(B - V) by Guetter & Vrba. Finally, we determined the distance modulus of Westerlund 2 to be V_0 - M_v = 13.9 ± 0.2 mag (d = 6.0 ± 0.6 kpc).