  Many intermediate age star clusters in the Magellanic Clouds present multiple
main sequence turn-offs (MMSTO), which challenge the classical idea that star
formation in such objects took place over short timescales. It has been
recently suggested that the presence of fast rotators among main sequence stars
could be the cause of such features (Bastian & de Mink 2009), hence relaxing
the need for extended periods of star formation. In this letter, we compute
evolutionary tracks and isochrones of models with and without rotation. We find
that, for the same age and input physics, both kinds of models present
turn-offs with an almost identical position in the colour-magnitude diagrams.
As a consequence, a dispersion of rotational velocities in coeval ensembles of
stars could not explain the presence of MMSTOs. We construct several synthetic
colour-magnitude diagrams for the different kinds of tracks and combinations of
them. The models that best reproduce the morphology of observed MMSTOs are
clearly those assuming a significant spread in the stellar ages - as long as
~400 Myr - added to a moderate amount of convective core overshooting. Only
these models produce the detailed "golf club" shape of observed MMSTOs. A
spread in rotational velocities alone cannot do anything similar. We also
discuss models involving a mixture of stars with and without overshooting, as
an additional scenario to producing MMSTOs with coeval populations. We find
that they produce turn-offs with a varying extension in the CMD direction
perpendicular to the lower main sequence, which are clearly not present in
observed MMSTOs.
