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We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtaindeep, high-resolution photometry of the young (age ~ 300 Myr) star clusterNGC1856 in the Large Magellanic Cloud. We compare the observed colour-magnitudediagram (CMD), after having applied a correction for differential reddening,with Monte Carlo simulations of simple stellar populations (SSPs) of variousages. We find that the main sequence turn-off (MSTO) region is wider than thatderived from the simulation of a single SSP. Using constraints based on thedistribution of stars in the MSTO region and the red clump, we find that theCMD is best reproduced using a combination of two different SSPs with agesseparated by 80 Myr (0.30 and 0.38 Gyr, respectively). However, we can notformally exclude that the width of the MSTO could be due to a range of stellarrotation velocities if the efficiency of rotational mixing is higher thantypically assumed. Using a King-model fit to the surface number density profilein conjunction with dynamical evolution models, we determine the evolution ofcluster mass and escape velocity from an age of 10 Myr to the present age,taking into account the possible effects of primordial mass segregation. Wefind that the cluster has an escape velocity Vesc ~ 17 km/s at an age of 10Myr, and it remains high enough during a period of ~ 100 Myr to retain materialejected by slow winds of first-generation stars. Our results are consistentwith the presence of an age spread in NGC1856, in contradiction to the resultsof Bastian & Silva-Villa (2013).

New Constraints on the Star Formation History of the Star Cluster NGC 1856