Zirconium, Barium, Lanthanum and Europium Abundances in Open Clusters
  We present an analysis of the s-process elements Zr, Ba, and La and the
r-process element Eu in a sample of 50 stars in 19 open clusters. Stellar
abundances of each element are based on measures of a minimum of two lines per
species via both equivalent width and spectrum synthesis techniques. We
investigate cluster mean neutron-capture abundance trends as a function of
cluster age and location in the Milky Way disk and compare them to results
found in other studies in the literature. We find a statistically significant
trend of increasing cluster [Ba/Fe] as a function of decreasing cluster age, in
agreement with recent findings for other open cluster samples, supporting the
increased importance of low-mass asymptotic giant branch stars to the
generation of s-process elements. However, the other s-process elements,
[La/Fe] and [Zr/Fe], do not show similar dependences, in contrast to
theoretical expectations and the limited observational data from other studies.
Conversely, cluster [Eu/Fe] ratios show a slight increase with increasing
cluster age, although with marginal statistical significance. Ratios of
[s/r]-process abundances, [Ba/Eu] and [La/Eu], however, show more clearly the
increasing efficiency of s-process relative to r-process enrichment in open
cluster chemical evolution, with significant increases among younger clusters.
Last, cluster neutron-capture element abundances appear to be independent of
Galactocentric distance. We conclude that a homogeneous analysis of a larger
sample of open clusters is needed to resolve the apparent discrepant
conclusions between different studies regarding s-process element abundance
trends with age to better inform models of galactic chemical evolution.
