Multicolour deep CCD photometric study of the moderately young southern open star clusters NGC 3228, NGC 4103, NGC 5662 and NGC 6087

Abstract

CCD observations in the U, B, V , R and I passbands have been used to generate colour- colour and deep colour-magnitude diagrams for the young open star clusters NGC 3228, NGC 4103, NGC 5662 and NGC 6087. The sample consists of about 2000 stars reaching down to V = 20 mag. The interstel- lar extinction across the face of the imaged cluster regions seems to be uniform with average values of E(B V )= 0 :31, 0.34 and 0.22 mag for NGC 4103, NGC 5662 and NGC 6087 respectively. The distances to the clusters NGC 3228, NGC 4103, NGC 5662 and NGC 6087 have been estimated as 0:5 0:05, 2:0 0:3, 0:8 0: 1a nd 1 : 0 0 :1 kpc respectively, while the corresponding ages estimated from the theoretical stellar evolutionary isochrones tted to the bright proper motion cluster mem- bers are 100, 30, 80 and 65 Myr respectively. Present data are consistent with the membership of the Cepheid variable V Cen to NGC 5662 and of S Norm to NGC 6087. Heavy contamination by foreground and background eld stars has unfortunately made it impossible to reach de- nite conclusions about the faint luminosity function, and the data do not reach faint enough limits to investigate pre-main sequence stars.