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An increasing number of young massive clusters (YMCs) in the MagellanicClouds have been found to exhibit bimodal or extended main sequences (MSs) intheir color--magnitude diagrams (CMDs). These features are usually interpretedin terms of a coeval stellar population with different stellar rotationalrates, where the blue and red MS stars are populated by non- (or slowly) andrapidly rotating stellar populations, respectively. However, some studies haveshown that an age spread of several million years is required to reproduce theobserved wide turn-off regions in some YMCs. Here we present theultraviolet--visual CMDs of four Large and Small Magellanic Cloud YMCs, NGC330, NGC 1805, NGC 1818, and NGC 2164, based on high-precision Hubble SpaceTelescope photometry. We show that they all exhibit extended main-sequenceturn-offs (MSTOs). The importance of age spreads and stellar rotation inreproducing the observations is investigated. The observed extended MSTOscannot be explained by stellar rotation alone. Adopting an age spread of 35--50Myr can alleviate this difficulty. We conclude that stars in these clusters arecharacterized by ranges in both their ages and rotation properties, but theorigin of the age spread in these clusters remains unknown.

Discovery of Extended Main Sequence Turn-offs in Four Young Massive Clusters in the Magellanic Clouds