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The most massive star clusters include several generations of stars with adifferent chemical composition (mainly revealed by an Na-O anti-correlation)while low-mass star clusters appear to be chemically homogeneous. We areinvestigating the chemical composition of several clusters with masses of a few10^4 Msun to establish the lower mass limit for the multiple stellar populationphenomenon. Using FLAMES@VLT spectra we determine abundances of Fe, O, Na, andseveral other elements (alpha, Fe-peak, and neutron-capture elements) in theold open cluster Berkeley 39. This is a massive open cluster: M~10^4 Msun,approximately at the border between small globular clusters and large openclusters. Our sample size of about 30 stars is one of the largest studied forabundances in any open cluster to date, and will be useful to determineimproved cluster parameters, such as age, distance, and reddening when coupledwith precise, well-calibrated photometry. We find that Berkeley 39 is slightlymetal-poor, <[Fe/H]>=-0.20, in agreement with previous studies of this cluster.More importantly, we do not detect any star-to-star variation in the abundancesof Fe, O, and Na within quite stringent upper limits. The r.m.s. scatter is0.04, 0.10, and 0.05 dex for Fe, O, and Na, respectively. This small spread canbe entirely explained by the noise in the spectra and by uncertainties in theatmospheric parameters. We conclude that Berkeley 39 is a single-populationcluster.

Searching for multiple stellar populations in the massive, old open cluster Berkeley 39