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

Recent studies have shown that the observed main-sequence turnoff (MSTO) in colour-magnitude diagrams of intermediate-age (1-2 Gyr) clusters in the Large Magellanic Cloud (LMC) is 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 the broadened MSTOs are not due to an age spread within the clusters.