Not-So-Simple Stellar Populations in the Intermediate-age Large
  Magellanic Cloud Star Clusters NGC 1831 and NGC 1868
  Using a combination of high-resolution Hubble Space Telescope/WFPC2
observations, we explore the physical properties of the stellar populations in
two intermediate-age star clusters in the Large Magellanic Cloud, NGC 1831 and
NGC 1868, based on their color-magnitude diagrams. We show that both clusters
exhibit extended main-sequence turn-offs. To explain the observations, we
consider variations in helium abundance, binarity, age dispersions, and fast
rotation of the clusters' member stars. The observed narrow main sequence
excludes significant variations in helium abundance in both clusters. We first
establish the clusters' main-sequence binary fractions using the bulk of the
clusters' main-sequence stellar populations >1 mag below their turn-offs. The
extent of the turn-off regions in color--magnitude space, corrected for the
effects of binarity, implies that age spreads of order 300 Myr may be inferred
for both clusters if the stellar distributions in color--magnitude space were
entirely due to the presence of multiple populations characterized by an age
range. Invoking rapid rotation of the population of cluster members
characterized by a single age also allows us to match the observed data in
detail. However, when taking into account the extent of the red clump in
color--magnitude space, we encounter an apparent conflict for NGC 1831 between
the age dispersion derived from that based on the extent of the main-sequence
turn-off and that implied by the compact red clump. We therefore conclude that,
for this cluster, variations in stellar rotation rate are preferred over an age
dispersion. For NGC 1868, both models perform equally well.
