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 } ~ { } M _ { \odot } 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 \sim 10 ^ { 4 } ~ { } M _ { \odot } , 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 .