The Intermediate-Age Globular Cluster NGC 1783 in the Large Magellanic Cloud

We present Hubble Space Telescope ACS deep photometry of the intermediate-age globular cluster NGC 1783 in the Large Magellanic Cloud. By using this photometric data set, we have determined the degree of ellipticity of the cluster (epsilon = 0.14 ± 0.03) and the radial density profile. This profile is well reproduced by a standard King model with an extended core (r_c = 24.5'') and a low concentration (c = 1.16), indicating that the cluster has not experienced the collapse of the core. We also derived the cluster age using the Pisa Evolutionary Library isochrones, with three different amounts of overshooting (namely, Λ_os = 0.0, 0.10, and 0.25). From a comparison of the observed color-magnitude diagram and main-sequence luminosity function (LF) with the theoretical isochrones and LFs, we find that only models with the inclusion of some overshooting (Λ_os = 0.10-0.25) are able to reproduce the observables. By using the magnitude difference δV = 0.90 between the mean level of the He clump and the flat region of the SGB, we derive an age τ = 1.4 ± 0.2 Gyr.