Searching for multiple stellar populations in the massive, old open
  cluster Berkeley 39
  The most massive star clusters include several generations of stars with a
different chemical composition (mainly revealed by an Na-O anti-correlation)
while low-mass star clusters appear to be chemically homogeneous. We are
investigating the chemical composition of several clusters with masses of a few
10^4 Msun to establish the lower mass limit for the multiple stellar population
phenomenon. Using FLAMES@VLT spectra we determine abundances of Fe, O, Na, and
several other elements (alpha, Fe-peak, and neutron-capture elements) in the
old open cluster Berkeley 39. This is a massive open cluster: M~10^4 Msun,
approximately at the border between small globular clusters and large open
clusters. Our sample size of about 30 stars is one of the largest studied for
abundances in any open cluster to date, and will be useful to determine
improved cluster parameters, such as age, distance, and reddening when coupled
with precise, well-calibrated photometry. We find that Berkeley 39 is slightly
metal-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 abundances
of Fe, O, and Na within quite stringent upper limits. The r.m.s. scatter is
0.04, 0.10, and 0.05 dex for Fe, O, and Na, respectively. This small spread can
be entirely explained by the noise in the spectra and by uncertainties in the
atmospheric parameters. We conclude that Berkeley 39 is a single-population
cluster.
