The colour magnitude diagrams (CMDs) of moderately young () open star clusters show the effect of stellar evolution at the
brighter end and contain pre-main sequence (PMS) stars at the fainter
end. Consequently, they provide a rare opportunity for reliable estimation of
ages of high and low mass stars formed in a molecular cloud. So they
can be used to determine the length of time over which star formation was
active in a particular region and also to know which stars form first -
high mass or low mass ones or if they formed together.
In order to address the questions raised above, we performed faint () multicolour CCD stellar photometry in five southern
moderately young open star clusters, namely NGC 3228, NGC 4103, NGC
4755, NGC 5662 and NGC 6087. Based on these observations, the presence
of PMS stars in NGC 4755 has been indicated for the first time by Sagar &
Cannon (1995). They also found that massive (
) and low
mass (
) stars in the cluster are
formed nearly at the same time from a molecular cloud which might have
existed for a period of about
. UBVRI photometry and similar
analyses for the other four clusters are presented here.
Table 1 (click here) lists the relevant prior information (taken from
Lyngå's (1987) catalogue) available on the clusters under
discussion. Some of these parameters should probably be revised to take
account of the new data presented here. The clusters all lie close to the
galactic plane ( and
) with moderate
interstellar extinction (
) and at a galactocentric
distance of
. Their ages and heliocentric distances range from
to 130 Myr
and 0.5 to 1.5 kpc respectively. The clusters under discussion are
relatively compact objects with angular radii
. NGC 5662
and NGC 6087 each have one Cepheid variable. NGC 4103 contains an
interesting eclipsing variable. All the clusters contain chemically
peculiar stars which may be due to their youthfulness.
Table 1: Preliminary informations about the clusters under
study. They are taken from the catalogue of Lyngå (1987)
Previous studies, the new observations and data reductions are described in the next three sections. The interstellar extinction, other photometric results and their implications for star formation processes are described in subsequent sections of this paper.