The search for multiple populations in Magellanic Cloud clusters -- IV. Coeval multiple stellar populations in the young star cluster NGC 1978

Abstract

We have recently shown that the $\\sim2$ Gyr old Large Magellanic Cloud star\ncluster NGC 1978 hosts multiple populations in terms of star-to-star abundance\nvariations in [N/Fe]. These can be seen as a splitting or spread in the\nsub-giant and red giant branches (SGB and RGB) when certain photometric filter\ncombinations are used. Due to its relative youth, NGC 1978 can be used to place\nstringent limits on whether multiple bursts of star-formation have taken place\nwithin the cluster, as predicted by some models for the origin of multiple\npopulations. We carry out two distinct analyses to test whether multiple\nstar-formation epochs have occurred within NGC 1978. First, we use UV CMDs to\nselect stars from the first and second population along the SGB, and then\ncompare their positions in optical CMDs, where the morphology is dominantly\ncontrolled by age as opposed to multiple population effects. We find that the\ntwo populations are indistinguishable, with age differences of $1\\pm20$ Myr\nbetween them. This is in tension with predictions from the AGB scenario for the\norigin of multiple populations. Second, we estimate the broadness of the main\nsequence turnoff (MSTO) of NGC 1978 and we report that it is consistent with\nthe observational errors. We find an upper limit of $\\sim$65 Myr on the age\nspread in the MSTO of NGC 1978. This finding is in conflict with the age spread\nscenario as origin of the extended MSTO in intermediate age clusters, while it\nfully supports predictions from the stellar rotation model.\n