Stars below in NGC 3228; below
in NGC 4103;
below
in NGC 5662 and below
in NGC 6087 show
a large spread in colour at a given brightness level, compared to that
expected from observational errors and differential reddening. The
main reasons for such a spread could be the presence of field stars;
binaries and peculiar cluster members. The field stars located in the
galactic plane, at about the cluster distances and with similar
reddenings, will occupy almost the same area in the colour-colour (CC)
and CM diagrams as the cluster members. It is therefore difficult to
separate cluster members from field stars only on the basis of their
closeness to the main populated area of the CM and CC diagrams.
However, the possibility of cluster membership is small for stars
located well away from the cluster MS in most of the CM and CC
diagrams. As the present observations are unable to quantify the field
star populations in the vicinity of clusters under study, the
statistically expected number of cluster members among the fainter
stars can not be determined in the same way as we did in NGC 4755
(Sagar & Cannon 1995). However, the following procedure has been
adopted for this purpose. The frequency distributions of stars are
derived from the
CM diagrams (see Table 12 (click here)). These
diagrams were preferred over other CM diagrams because of
better data completeness in them. Inspection of our CCD frames and an
estimate of the numbers of crowded images indicate that we are
85% complete in general. To avoid the effect of relatively
large (
50%) data incompleteness, the analysis was restricted
to the brightness level which is about 1 mag above the limiting
magnitude of our observations. To derive the frequency distribution of
stars, the
CM diagram is divided into two magnitude bins, namely
brighter and fainter; and into three colour bins called ``below", ``near"
and
``above MS". The brighter magnitude bin represents the region in which
the cluster main sequences are clearly visible, while in the fainter
magnitude bin, any cluster sequence is hardly distinguishable from the
field star contamination. Stars lying within two standard deviations
in
either side of the ZAMS are put in the colour bin called
``near MS" and
denotes number of such stars in
Table 12 (click here). The width of the colour bins increases with decreasing
brightness. Stars located towards the blue and red sides of this bin are put
in the colour bins named ``below MS" and ``above MS" respectively.
and
denote the corresponding numbers in
Table 12 (click here). This table clearly shows that:
Table 12: Frequency distribution of the stars in the ,
diagram for the clusters under study derived from our sample.
,
and
denote number of stars below, near
and above the ZAMS respectively while
denotes number of cluster
members expected in the fainter magnitude bin assuming Salpeter mass function
slope
Figure 17: The luminosity functions of the ``near MS"
() stars and the expected cluster
members (
) are shown by solid and dotted
histograms respectively. N is the number of stars in
one magnitude bin of
Figure 18: The ,
diagrams are plotted again in
a), b), c) and d) for NGC 3228, NGC 4103, NGC 5662 and
NGC 6087 respectively. Theoretical stellar evolutionary isochrones taken from
Iben (1965) for the pre-MS phase of evolution are plotted. The
theoretical stellar birthlines taken from Palla & Stahler (1992) are
shown by dotted lines