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 ± 0.1 and 1.0 ± 0.1 kpc respectively, while the corresponding ages estimated from the theoretical stellar evolutionary isochrones fitted 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 field stars has unfortunately made it impossible to reach defi- nite conclusions about the faint luminosity function, and the data do not reach faint enough limits to investigate pre-main sequence stars.