A multicolour CCD photometric and mass function study of the distant southern open star clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253

Abstract

We derive cluster parameters and mass functions from new UBVRI CCD photometric observations of ,3500 stars reaching down to V , 20 mag for the distant southern open star clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253. For NGC 3105 and Hogg 15, CCD data are presented for the first time. The reddening is nonuniform across the face of the young (age ,300 Myr) clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15 and Pismis 20, with average values of EB 2 V 1:06; 1.44, 0.52, 1.15 and 1.20 mag respectively, while it is uniform with average values of EB 2 V 0:72 and 0.20 mag for the older clusters NGC 4815 and 6253 respectively. The values of colour excess ratios indicate the presence of normal interstellar reddening across the cluster regions studied here. Well-defined main sequences can be seen in all the clusters. However, mainsequence turn-off points and subgiant branches are well defined only in the older clusters NGC 4815 and 6253. The distances to the clusters