New Constraints on the Star Formation History of the Star Cluster NGC
  1856
  We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtain
deep, high-resolution photometry of the young (age ~ 300 Myr) star cluster
NGC1856 in the Large Magellanic Cloud. We compare the observed colour-magnitude
diagram (CMD), after having applied a correction for differential reddening,
with Monte Carlo simulations of simple stellar populations (SSPs) of various
ages. We find that the main sequence turn-off (MSTO) region is wider than that
derived from the simulation of a single SSP. Using constraints based on the
distribution of stars in the MSTO region and the red clump, we find that the
CMD is best reproduced using a combination of two different SSPs with ages
separated by 80 Myr (0.30 and 0.38 Gyr, respectively). However, we can not
formally exclude that the width of the MSTO could be due to a range of stellar
rotation velocities if the efficiency of rotational mixing is higher than
typically assumed. Using a King-model fit to the surface number density profile
in conjunction with dynamical evolution models, we determine the evolution of
cluster mass and escape velocity from an age of 10 Myr to the present age,
taking into account the possible effects of primordial mass segregation. We
find that the cluster has an escape velocity Vesc ~ 17 km/s at an age of 10
Myr, and it remains high enough during a period of ~ 100 Myr to retain material
ejected by slow winds of first-generation stars. Our results are consistent
with the presence of an age spread in NGC1856, in contradiction to the results
of Bastian & Silva-Villa (2013).
