Age of an open cluster is obtained by fitting theoretical isochrones to its colour - magnitude diagrams. This requires however, a knowledge of cluster metallicity and reddening. Distance modulus can be determined simultaneously with age. The difficulties encountered in fitting theoretical isochrones to observed cluster CMDs when no information about reddening and/or metallicity is available, have prompted some indirect methods for age estimation of open clusters. These methods use the visible differences in the CMDs of star clusters for different ages (see e.g., Kaluzny 1994; Phelps et al. 1994), the position of red giant clump, main sequence blue turnoff (BTO) and red turnoff point.
We have used another colour index parameter (CIP) which is the difference in colour index between the blue turnoff (BTO) point of the main sequence and the colour at the base of the red giant branch (BRGB). This parameter can be useful in clusters having no noticeable clump. This parameter slightly depends on metallicity but still can be used to have an estimate of age of intermediate age clusters. The variation of the colour index parameter with age has been shown in Figs. 4 (click here)a and 4b. The data have been taken from VandenBerg (1985) and Bertelli et al. (1994).
Figure 5: The CMDs for all stars in the Be 64 region. The stars lying in the box in (V, V-I) CMD mainly belong to the foreground population (see text)
Figure 6: The CMDs for two nearby fields towards east (open circles) and west (filled circles) directions at away from the cluster Be 64
Figure 7: The CMDs for all stars in Be 69 region
Figure 8: The CMDs for two nearby fields towards north (open circles) and south (filled circles) directions at away from the cluster Be 69