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 Magellanic Cloud cluster NGC1978, recently confirmed to harbor multiple populations of nearly the same age. We used accurate Stromgren photometry of carefully selected cluster red giant branch stars along with a high-dispersion spectroscopy-based calibration of the metallicity-sensitive index m1. Once accounting for the photometry quality, assessed from extensive artificial stars tests to trace the photometric uncertainties as a function of the position to the cluster's center as well as the stellar brightness, and those from the metallicity calibration, we found that NGC1978 exhibits a small metallicity spread of 0.035 dex (+- 0.019-0.023), depending on whether stars with individual sigma[Fe/H] < 0.15 dex or those located in the cluster's outer areas are considered. Such a spread in [Fe/H] is consistent with a cluster formation model with self-enrichment, if mass loss higher than 90 per cent due to stellar evolutionary and galactic tidal effects is assumed. Nevertheless, scenarios where the apparent [Fe/H] variation reflects CN abundance anomalies or less extreme mass loss models with environmentally-dependent self enrichment should not be ruled out.

11 pages, 4 figures. Accepted for publication in The Astronomical Journal

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• Metallicities and ages for star clusters and their surrounding fields in the Large Magellanic Cloud
• Rotational Variation Allows for Narrow Age Spread in the Extended Main Sequence Turnoff of Massive Cluster NGC 1846
• Strömgren metallicities for intermediate-age and old star clusters
• Multiple populations of Hbeta emission line stars in the Large Magellanic Cloud cluster NGC1971