A&A Supplement series, Vol. 121, April I 1997, 9-29
Received April 2; accepted June 26, 1996
R. Sagar
- R.D.
Cannon
Send offprint request: R. Sagar
Indian Institute of Astrophysics, Bangalore 560034,
India
e-mail address: sagar@upso.ernet.in
Anglo-Australian Observatory, P.O. Box 296,
Epping NSW 2121, Australia
e-mail address: rdc@aaoepp.aao.gov.au
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 
.
The interstellar 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 
, 
, 
and
kpc respectively, while the corresponding ages estimated from
the theoretical stellar evolutionary isochrones fitted to the bright proper
motion cluster members are 
, 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 definite conclusions about
the faint luminosity function, and the data do not reach faint
enough limits to investigate pre-main sequence stars.
keywords: HR diagram -- stars: evolution -- open clusters and associations: individual: NGC 3228, NGC 4103, NGC 5662, NGC 6087