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We discuss new photometry from high-resolution images of 7 intermediate-age(1-2 Gyr) star clusters in the Large Magellanic Cloud taken with the AdvancedCamera for Surveys on board the Hubble Space Telescope. We fit color-magnitudediagrams (CMDs) with several different sets of theoretical isochrones, anddetermine systematic uncertainties for population parameters when derived usingany one set of isochrones. The cluster CMDs show several interesting features,including extended main sequence turnoff (MSTO) regions, narrow red giantbranches, and clear sequences of unresolved binary stars. We show that theextended MSTOs are not caused by photometric uncertainties, contamination byfield stars, or the presence of binary stars. Enhanced helium abundances in afraction of cluster stars are also ruled out as the reason for the extendedMSTOs. Quantitative comparisons with simulations indicate that the MSTO regionsare better described by a spread in ages than by a bimodal age distribution,although we can not formally rule out the latter for the three lowest-massclusters in our sample (which have masses lower than about 3E4 solar masses).This conclusion differs from that of some previous works which suggested thatthe age distribution in massive clusters in our sample is bimodal. Thissuggests that any secondary star formation occurred in an extended fashionrather than through short bursts. We discuss these results in the context ofthe nature of multiple stellar populations in star clusters.

astrophysical journal/the astrophysical journal

Population Parameters of Intermediate-Age Star Clusters in the Large  Magellanic Cloud. II. New Insights from Extended Main Sequence Turnoffs in 7  Star Clusters