NGC 1866: a milestone for understanding the chemical evolution of stellar populations in the Large Magellanic Cloud★

We present new FLAMES@VLT spectroscopic observations of 30 stars in the field of the Large Magellanic Cloud (LMC) stellar cluster NGC 1866. NGC 1866 is one of the few young and massive globular clusters that is close enough so that its stars can be individually studied in detail. Radial velocities have been used to separate stars belonging to the cluster and to the LMC field, and the same spectra have been used to derive chemical abundances for a variety of elements, from [Fe/H] to the light (i.e. Na, O, Mg, etc.) to the heavy ones. The average iron abundance of NGC 1866 turns out to be [Fe/H]=-0.43 ± 0.01 dex (with a dispersion σ= 0.04 dex), from the analysis of 14 cluster member stars. Within our uncertainties, the cluster stars are homogeneous, as far as chemical composition is concerned, independent of the evolutionary status. The observed cluster stars do not show any sign of the light elements' 'anticorrelation' present in all the Galactic globular clusters so far studied and are also found in the old LMC stellar clusters. A similar lack of anticorrelations has been detected in the massive intermediate-age LMC clusters, indicating a different formation/evolution scenario for the LMC massive clusters younger than ∼3 Gyr with respect to the old ones. Also opposite to the Galactic globulars, the chemical composition of the older red giant branch field stars and of the young post-main-sequence cluster stars show robust homogeneity suggesting a quite similar process of chemical evolution. The field and cluster abundances are in agreement with recent chemical analysis of LMC stars, which show a distinctive chemical pattern for this galaxy with respect to the Milky Way. We discuss these findings in light of the theoretical scenario of chemical evolution of the LMC.