Multiple stellar populations in Magellanic Cloud clusters. VI. A survey
  of multiple sequences and Be stars in young clusters
  The split main sequences (MSs) and extended MS turnoffs (eMSTOs) detected in
a few young clusters have demonstrated that these stellar systems host multiple
populations differing in a number of properties such as rotation and, possibly,
age.We analyze Hubble Space Telescope photometry for thirteen clusters with
ages between ~40 and ~1000 Myrs and of different masses. Our goal is to
investigate for the first time the occurrence of multiple populations in a
large sample of young clusters. We find that all the clusters exhibit the eMSTO
phenomenon and that MS stars more massive than ~1.6 solar masses define a blue
and red MS, with the latter hosting the majority of MS stars. The comparison
between the observations and isochrones suggests that the blue MSs are made of
slow-rotating stars, while the red MSs host stars with rotational velocities
close to the breakup value. About half of the bright MS stars in the youngest
clusters are H-alpha emitters. These Be stars populate the red MS and the
reddest part of the eMSTO thus supporting the idea that the red MS is made of
fast rotators. We conclude that the split MS and the eMSTO are a common feature
of young clusters in both Magellanic Clouds. The phenomena of a split MS and an
eMSTO occur for stars that are more massive than a specific threshold which is
independent of the host-cluster mass. As a by-product, we report the
serendipitous discovery of a young SMC cluster, GALFOR1.
