Search for a Metallicity Spread in the Multiple Population Large Magellanic Cloud Cluster NGC 1978

Abstract

We report on the spread of [Fe/H] values in the massive Large MagellanicCloud cluster NGC1978, recently confirmed to harbor multiple populations ofnearly the same age. We used accurate Stromgren photometry of carefullyselected cluster red giant branch stars along with a high-dispersionspectroscopy-based calibration of the metallicity-sensitive index m1. Onceaccounting for the photometry quality, assessed from extensive artificial starstests to trace the photometric uncertainties as a function of the position tothe cluster´s center as well as the stellar brightness, and those from themetallicity calibration, we found that NGC1978 exhibits a small metallicityspread of 0.035 dex (+- 0.019-0.023), depending on whether stars withindividual sigma[Fe/H] < 0.15 dex or those located in the cluster´s outer areasare considered. Such a spread in [Fe/H] is consistent with a cluster formationmodel with self-enrichment, if mass loss higher than 90 per cent due to stellarevolutionary and galactic tidal effects is assumed. Nevertheless, scenarioswhere the apparent [Fe/H] variation reflects CN abundance anomalies or lessextreme mass loss models with environmentally-dependent self enrichment shouldnot be ruled out.