4 Ages of cluster members

Udalski et al. (1998) provided a BVI photometric survey of the SMC central $\approx$ 2.4 square degrees with results for about 2.2 million stars. The data were collected during the OGLE-II microlensing search project (Udalski et al. 1997).

Pietrzynski & Udalski (1999b) studied colour-magnitude diagrams (CMDs) extracted from the BVI database for 93 SMC star clusters, taking into account neighbouring field extractions for comparisons. They estimated reddening values for each cluster using: (i) the mean I band magnitude of red clump stars in the cluster neighbourhood; (ii) the assumed extinction-free magnitude of the SMC's red clump stars, I = 18.34 mag (Udalski 1998). They derived ages using Bertelli et al.'s (1994) isochrones, adopting as a rule for the SMC a metallicity Z = 0.004, and an absolute distance modulus (m-M) = 18.65.

Considering the OGLE-II angular coverage we conclude that out of the present 176 objects 133 are therein included. We obtained from the database V and I band CMDs for these objects, using their coordinates and diameters to define a box extraction. Guided by DSS images of each system we selected representative field regions to extract stars and construct CMDs for comparison purposes. Ages and reddenings for the clusters were estimated by fitting the Padova isochrones. We adopted Bertelli et al.'s (1994) isochrones rather than the new isochrones by Girardi et al. (2000), since the previous set includes younger ages. The isochrone age grid suitable for 4, 5, 6.3, 7.9, 10, 12, 16, 20, 25, 32, 40, 50, 63, 79, 100, 126, 160, 199, 251, 316, 400, 500, 630, 790 and 1000 Myr. The isochrone age range and resolution allow one to estimate errors taking into account the stellar statistics in the cluster and field CMDs. We also assumed a SMC metallicity of Z = 0.004, a foreground galactic reddening of $E(B-V)_{\rm f} = 0.03$ in the SMC direction and E(V-I)/E(B-V) = 1.31. Table 2 shows the ages, in Col. 2 and reddening values (foreground plus SMC internal) in Col. 3 determined in the present work via isochrone fitting. Ages and reddening values derived by Pietrzynski & Udalski (1999b) are in Cols. 4 and 5 respectively, when available for comparisons. In our sample we could estimate CMD ages for 91 objects, 40 of them in common with Pietrzynski & Udalski (1999b). We conclude that despite the reddening method and distance modulus differences between the two studies, there is good overall agreement for the age determinations. We also included ages for 21 objects embedded in HII regions, classified as NA and NC (see Col. 4 of Table 1, and also BS95). We assumed for them an age of 3 Myr.