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To see whether the present observations have reached to the faintness
level of PMS stars or not, the
,
diagrams for the
stars observed by us (filled circles), as well as for the members (generally
proper motion) observed photoelectrically by others (crosses) but not present
in our sample have been plotted in Fig. 18 (click here) for all the star
clusters under study. In plotting this figure, we used uniform interstellar
extinction with values of E(B-V) = 0.03, 0.31, 0.34 and 0.22 for NGC 3228,
NGC 4103, NGC 5662 and NGC 6087 respectively. The corresponding values used
for their true distance moduli are 8.5, 11.5, 9.5 and 9.9. In this diagram,
we have also plotted Iben's (1965) isochrones for contracting
stars with ages 10 and 30 Myr and the theoretical stellar birthline given
by Palla & Stahler (1992), which marks the luminosity and
temperature at which contracting stars are expected first to become visible at
optical wavelengths, together with the ZAMS. The following inferences
can be drawn from this figure:
- (i)
- A few stars in NGC 4103 and NGC 6087 are located well
above the theoretical stellar birthline. They are probably field stars.
- (ii)
- No PMS is clearly visible in the clusters under
study. The low-mass stars which formed at the same time as the more
massive stars are expected to join the ZAMS at
, +4.1,
+6.0 and +5.8 for the clusters NGC 3228, NGC 4103, NGC 5662 and NGC
6087 respectively. So the present observations have barely reached to
the faintness level for detecting the PMS stars. Also to make sure,
one should look for signatures of PMS stars like
emission, irregular variability or ultraviolet excess in them, as
Adams et al. (1983) did in NGC 2264. On the other hand, our
observations in NGC 4755 have been able to detect PMS stars (see Sagar
& Cannon 1995). NGC 4755 is the youngest (
Myr) open star
cluster observed under this programme. The clearly visible kink away
from the ZAMS around
in
,
diagram of NGC
4755 (see Fig. 7 (click here) in Sagar & Cannon 1995) is
interpreted as the MS turn-on for PMS stars. Most of the PMS stars have an
age around 6 Myr, indicating that massive stars in NGC 4755 formed about 4
Myr before the bulk of the low-mass stars.

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