Exploring the s-process history in the Galactic disk: Cerium abundances
  and gradients in Open Clusters from the OCCAM/APOGEE sample
  The APOGEE Open Cluster Chemical Abundances and Mapping (OCCAM) survey is
used to probe the chemical evolution of the s-process element cerium in the
Galactic disk. Cerium abundances were derived from measurements of Ce II lines
in the APOGEE spectra using the Brussels Automatic Code for Characterizing High
Accuracy Spectra (BACCHUS) in 218 stars belonging to 42 open clusters. Our
results indicate that, in general, for Ages $<$ 4 Gyr, younger open clusters
have higher [Ce/Fe] and [Ce/$\alpha$-element] ratios than older clusters. In
addition, metallicity segregates open clusters in the [Ce/X]-Age plane (where X
can be H, Fe, and the $\alpha$-elements O, Mg, Si, or Ca). These
metallicity-dependant relations result in [Ce/Fe] and [Ce/$\alpha$] ratios with
age that are not universal clocks. Radial gradients of [Ce/H] and [Ce/Fe]
ratios in open clusters, binned by age, were derived for the first time, with
d[Ce/H]dR$_{GC}$ being negative, while d[Ce/Fe]/dR$_{GC}$ is positive. [Ce/H]
and [Ce/Fe] gradients are approximately constant over time, with the [Ce/Fe]
gradient becoming slightly steeper, changing by $\sim$+0.009
dex-kpc$^{-1}$-Gyr$^{-1}$. Both the [Ce/H] and [Ce/Fe] gradients are shifted to
lower values of [Ce/H] and [Ce/Fe] for older open clusters. The chemical
pattern of Ce in open clusters across the Galactic disk is discussed within the
context of s-process yields from AGB stars, $\sim$Gyr time delays in Ce
enrichment of the interstellar medium, and the strong dependence of Ce
nucleosynthesis on the metallicity of its AGB stellar sources.
