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Using a combination of high-resolution Hubble Space Telescope/WFPC2observations, we explore the physical properties of the stellar populations intwo intermediate-age star clusters in the Large Magellanic Cloud, NGC 1831 andNGC 1868, based on their color-magnitude diagrams. We show that both clustersexhibit extended main-sequence turn-offs. To explain the observations, weconsider variations in helium abundance, binarity, age dispersions, and fastrotation of the clusters' member stars. The observed narrow main sequenceexcludes significant variations in helium abundance in both clusters. We firstestablish the clusters' main-sequence binary fractions using the bulk of theclusters' main-sequence stellar populations >1 mag below their turn-offs. Theextent of the turn-off regions in color--magnitude space, corrected for theeffects of binarity, implies that age spreads of order 300 Myr may be inferredfor both clusters if the stellar distributions in color--magnitude space wereentirely due to the presence of multiple populations characterized by an agerange. Invoking rapid rotation of the population of cluster memberscharacterized by a single age also allows us to match the observed data indetail. However, when taking into account the extent of the red clump incolor--magnitude space, we encounter an apparent conflict for NGC 1831 betweenthe age dispersion derived from that based on the extent of the main-sequenceturn-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 agedispersion. For NGC 1868, both models perform equally well.

Not-So-Simple Stellar Populations in the Intermediate-age Large Magellanic Cloud Star Clusters NGC 1831 and NGC 1868