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

TL;DR

This study investigates the chemical homogeneity of the old open cluster Berkeley 39, finding no evidence of multiple stellar populations, thus suggesting it is a single-population cluster despite its high mass.

Contribution

First detailed chemical abundance analysis of Berkeley 39, establishing its status as a single-population cluster at the mass boundary between open and globular clusters.

Findings

No star-to-star variation in Fe, O, Na within measurement uncertainties

Berkeley 39 is slightly metal-poor with <[Fe/H]>=-0.20

Cluster is confirmed as a single-population system

Abstract

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.