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Constraints on possible age spreads within young massive clusters in the Large Magellanic Cloud

Recent studies have shown that the observed main-sequence turn-off (MSTO) in colour-magnitude diagrams of intermediate age (1-2 Gyr) clusters in the LMC are broader than would be nominally expected for a simple stellar population. This has led to the suggestion that such clusters may host multiple stellar populations, with age spreads of 100-500 Myr. However, at intermediate ages, spreads of this magnitude are difficult to discern and alternative explanations have been put forward (e.g., stellar rotation, interacting binaries). A prediction of the age-spread scenario is that younger clusters in the LMC, with similar masses and radii, should also show significant age spreads. In younger clusters (i.e., 40-300 Myr) such large age spreads should be readily apparent. We present an analysis of the colour-magnitude diagrams of two massive young clusters in the LMC (NGC 1856 and NGC 1866) and show that neither have such large age spreads, in fact, both are consistent with a single burst of star-formation (sigma(age) < 35 Myr). This leads us to conclude that either the intermediate age clusters in the LMC are somehow special or that the broadened MSTOs are not due to an age spread within the clusters.