A deep UBVRI CCD photometric study of the moderately young southern open star cluster NGC 4755 = Kappa-Crucis

Abstract

CCD observations in U, B, V, R and I passbands have been used to generate (U-B), (B-V) colour-colour and deep V, (B-V); V, (U-B) and V, (V-I) colour-magnitude diagrams for the young open cluster NGC 4755. The sample consists of 813 stars reaching down to V~20mag. There appears to be non-uniform extinction over the face of the cluster with a mean value of E(B-V)=0.41mag and a range of about 0.05mag. The true distance modulus to the cluster has been estimated as 11.6+/-0.2mag. An age of ~10Myr has been assigned to the post-main sequence (MS) cluster members. Ages of the post-MS and pre-MS stars indicate that massive (M>10Msun_) and low mass (M<2Msun_) stars in the cluster are formed nearly at the same time from a molecular cloud which might have existed for a minimum period of about 6-7Myr.