Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. III. Padova Results

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Abstract
The color-magnitude diagrams (CMDs) of three intermediate-age Large Megallanic Cloud clusters, NGC 2173, SL 556, and NGC 2155, are analyzed to determine their age and metallicity based on Padova stellar models. Synthetic CMDs are compared with cluster data. The best match is obtained using two fitting functions based on star counts in the different bins of the cluster CMD. Two different criteria are used. One of them takes into account the uncertainties in the color of the red clump stars. Given the uncertainties on the experimental values of the clusters metallicity, we provide a set of acceptable solutions. They define the corresponding values of metallicity, age, reddening and distance modulus (for the assumed initial mass function). The comparison with Padova models suggests for NGC 2173 a prolonged star formation (spanning a period of about 0.3 Gyr), beginning 1.7 Gyr and ending 1.4 Gyr ago. The metallicity Z is in the range 0.0016–0.003. For SL 556 an age of 2.0 Gyr is obtained. The metallicity value is in the range 0.002–0.004, depending on the adopted comparison criterion. The derived age for NGC 2155 is 2.8 Gyr, and its metallicity Z is in the range 0.002–0.003. The CMD features of this cluster suggest that a more efficient overshoot should be adopted in the evolutionary models. A period of extended star formation is not required to fit the SL 556 and NGC 2155 observations.
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