Population Parameters of Intermediate-Age Star Clusters in the Large
  Magellanic Cloud. II. New Insights from Extended Main Sequence Turnoffs in 7
  Star Clusters
  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 Advanced
Camera for Surveys on board the Hubble Space Telescope. We fit color-magnitude
diagrams (CMDs) with several different sets of theoretical isochrones, and
determine systematic uncertainties for population parameters when derived using
any one set of isochrones. The cluster CMDs show several interesting features,
including extended main sequence turnoff (MSTO) regions, narrow red giant
branches, and clear sequences of unresolved binary stars. We show that the
extended MSTOs are not caused by photometric uncertainties, contamination by
field stars, or the presence of binary stars. Enhanced helium abundances in a
fraction of cluster stars are also ruled out as the reason for the extended
MSTOs. Quantitative comparisons with simulations indicate that the MSTO regions
are 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-mass
clusters in our sample (which have masses lower than about 3E4 solar masses).
This conclusion differs from that of some previous works which suggested that
the age distribution in massive clusters in our sample is bimodal. This
suggests that any secondary star formation occurred in an extended fashion
rather than through short bursts. We discuss these results in the context of
the nature of multiple stellar populations in star clusters.
