The Effect of Stellar Rotation on Colour-Magnitude Diagrams: On the
  apparent presence of multiple populations in intermediate age stellar
  clusters
  A significant number of intermediate age clusters (1-2 Gyr) in the Magellanic
Clouds appear to have multiple stellar populations within them, derived from
bi-modal or extended main sequence turn offs. If this is interpreted as an age
spread, the multiple populations are separated by a few hundred Myr, which
would call into question the long held notion that clusters are simple stellar
populations. Here we show that stellar rotation in stars with masses between
1.2-1.7 Msun can mimic the effect of a double or multiple population, whereas
in actuality only a single population exists. The two main causes of the spread
near the turn-off are the effects of stellar rotation on the structure of the
star and the inclination angle of the star relative to the observer. Both
effects change the observed effective temperature, hence colour, and flux of
the star. In order to match observations, the required rotation rates are
20-50% of the critical rotation, which are consistent with observed rotation
rates of similar mass stars in the Galaxy. We provide scaling relations which
can be applied to non-rotating isochrones in order to mimic the effects of
rotation. Finally, we note that rotation is unlikely to be the cause of the
multiple stellar populations observed in old globular clusters, as low mass
stars (<1 Msun) are not expected to be rapid rotators.
