A Young Cluster With an Extended Main Sequence Turnoff: Confirmation of
  a Prediction of the Stellar Rotation Scenario
  We present Hubble Space Telescope photometry of NGC1850, a ~100 Myr, ~10^5
Msun cluster in the Large Magellanic Cloud. The colour magnitude diagram
clearly shows the presence of an extended main sequence turnoff (eMSTO). The
use of non-rotating stellar isochrones leads to an age spread of ~40 Myr. This
is in good agreement with the age range expected when the effects of rotation
in MSTO stars are wrongly interpreted in terms of age spread. We also do not
find evidence for multiple, isolated episodes of star-formation bursts within
the cluster, in contradiction to scenarios that invoke actual age spreads to
explain the eMSTO phenomenon. NGC 1850 therefore continues the trend of eMSTO
clusters where the inferred age spread is proportional to the age of the
cluster. While our results confirm a key prediction of the scenario where
stellar rotation causes the eMSTO feature, direct measurements of the
rotational rate of MSTO stars is required to definitively confirm or refute
whether stellar rotation is the origin of the eMSTO phenomenon or if it is due
to an as yet undiscovered effect.
