In the past few years the comparison of stellar evolutionary models with observations of old open clusters has become interesting because the CMD of old open clusters might be quite sensitive to the effects of convective overshoot mixing. Now it is possible to compare models computed without overshoot e.g., those by VandenBerg (1985) with models incorporating overshoot (e.g., Bertelli et al. 1994). Age and distance of the cluster can also be obtained by fitting theoretical isochrones to the CMDs. The major problem in determining the age and distance modulus by comparing the theoretical isochrones with the observed CMDs, is the absence of data about the metal abundance [Fe/H]. Therefore we have used isochrones of different metallicity and tried to find out which provides the best global fit.
The clean CMDs of Be 64 after removing contribution of probable
foreground stars have been used to compare with the theoretical isochrones. The
best comparison with the standard evolutionary models of VandenBerg
(1985), shown in Fig. 11 (click here), yields an age 
and
(m-M)= 16.0 for Z= 0.01. The comparison in
Fig. 11 (click here) is rather
difficult to interpret because of scatter in the turnoff region. The comparison of
CMDs of Be 69 with the standard models of VandenBerg (1985) is shown
in Fig. 12 (click here). The best comparison yields an age of 
and (m-M)= 14.0 for Z= 0.006. However, in case of Be 69, there seems an apparent
discrepancy between the shape of turnoff and the isochrones. This discrepancy has
been noted before by several authors (e.g; Anthony-Twarog et al. 1991;
Alfaro et al. 1992) and can be explained as a failure of the standard
isochrones to include convective overshoot.
Theoretical isochrones with convective overshoot by Bertelli et al.
(1994) have also been compared with the CMDs of Be 64 and Be 69 in
Figs. 13 (click here) and 14 (click here). The best fit is obtained for Z= 0.008, age
for both the clusters. The apparent distance modulus
(m-M) comes out to be 16.2 (distance 
) and 14.3
(distance 
) for Be 64 and Be 69, respectively. The errors
in distance estimates are the combination of errors in interstellar extinction
and in visual fitting of the isochrones to the observations. The morphological
features of the CMDs match very well with the theoretical isochrones predicted by
the Bertelli et al. (1994). However, to discriminate between different
evolutionary scenarios photometry with membership determination and spectroscopic
determination of metallicity are required.
Figure 15: The spatial distribution of the evolved stars in the
cluster Be 64. The cross represents adopted center of the cluster
Figure 16: Same as Fig. 15 but for Be 69