The search for multiple populations in Magellanic Cloud Clusters IV:
  Coeval multiple stellar populations in the young star cluster NGC 1978
  We have recently shown that the $\sim2$ Gyr old Large Magellanic Cloud star
cluster NGC 1978 hosts multiple populations in terms of star-to-star abundance
variations in [N/Fe]. These can be seen as a splitting or spread in the
sub-giant and red giant branches (SGB and RGB) when certain photometric filter
combinations are used. Due to its relative youth, NGC 1978 can be used to place
stringent limits on whether multiple bursts of star-formation have taken place
within the cluster, as predicted by some models for the origin of multiple
populations. We carry out two distinct analyses to test whether multiple
star-formation epochs have occurred within NGC 1978. First, we use UV CMDs to
select stars from the first and second population along the SGB, and then
compare their positions in optical CMDs, where the morphology is dominantly
controlled by age as opposed to multiple population effects. We find that the
two populations are indistinguishable, with age differences of $1\pm20$ Myr
between them. This is in tension with predictions from the AGB scenario for the
origin of multiple populations. Second, we estimate the broadness of the main
sequence turnoff (MSTO) of NGC 1978 and we report that it is consistent with
the observational errors. We find an upper limit of $\sim$65 Myr on the age
spread in the MSTO of NGC 1978. This finding is in conflict with the age spread
scenario as origin of the extended MSTO in intermediate age clusters, while it
fully supports predictions from the stellar rotation model.
