Extended Main Sequence Turnoffs in Intermediate-Age Star Clusters:
  Stellar Rotation diminishes, but does not eliminate, Age Spreads
  Extended main sequence turn-off (eMSTO) regions are a common feature in
color-magnitude diagrams of young and intermediate-age star clusters in the
Magellanic Clouds. The nature of eMSTOs remains debated in the literature. The
currently most popular scenarios are extended star formation activity and
ranges of stellar rotation rates. Here we study details of differences in MSTO
morphology expected from spreads in age versus spreads in rotation rates, using
Monte Carlo simulations with the Geneva SYCLIST isochrone models that include
the effects of stellar rotation. We confirm a recent finding of Niederhofer et
al. that a distribution of stellar rotation velocities yields an MSTO extent
that is proportional to the cluster age, as observed. However, we find that
stellar rotation yields MSTO crosscut widths that are generally smaller than
observed ones at a given age. We compare the simulations with high-quality
Hubble Space Telescope data of NGC 1987 and NGC 2249, which are the two only
relatively massive star clusters with an age of ~1 Gyr for which such data is
available. We find that the distribution of stars across the eMSTOs of these
clusters cannot be explained solely by a distribution of stellar rotation
velocities, unless the orientations of rapidly rotating stars are heavily
biased towards an equator-on configuration. Under the assumption of random
viewing angles, stellar rotation can account for ~60% and ~40% of the observed
FWHM widths of the eMSTOs of NGC 1987 and NGC 2249, respectively. In contrast,
a combination of distributions of stellar rotation velocities and stellar ages
fits the observed eMSTO morphologies very well.
