Discovery of Extended Main Sequence Turn-offs in Four Young Massive
  Clusters in the Magellanic Clouds
  An increasing number of young massive clusters (YMCs) in the Magellanic
Clouds have been found to exhibit bimodal or extended main sequences (MSs) in
their color--magnitude diagrams (CMDs). These features are usually interpreted
in terms of a coeval stellar population with different stellar rotational
rates, where the blue and red MS stars are populated by non- (or slowly) and
rapidly rotating stellar populations, respectively. However, some studies have
shown that an age spread of several million years is required to reproduce the
observed wide turn-off regions in some YMCs. Here we present the
ultraviolet--visual CMDs of four Large and Small Magellanic Cloud YMCs, NGC
330, NGC 1805, NGC 1818, and NGC 2164, based on high-precision Hubble Space
Telescope photometry. We show that they all exhibit extended main-sequence
turn-offs (MSTOs). The importance of age spreads and stellar rotation in
reproducing the observations is investigated. The observed extended MSTOs
cannot be explained by stellar rotation alone. Adopting an age spread of 35--50
Myr can alleviate this difficulty. We conclude that stars in these clusters are
characterized by ranges in both their ages and rotation properties, but the
origin of the age spread in these clusters remains unknown.
