Abundances of disk and bulge giants from high-resolution optical spectra -- IV. Zr, La, Ce, Eu

Abstract

Stellar mass and metallicity are factors that affect the neutron-capture process. Due to this, the enrichment of the ISM and the abundance of neutron-capture elements vary with time, making them suitable probes for the Galactic chemical evolution. In this work we make a differential comparison of neutron-capture element abundances determined in the local disk(s) and the bulge, focusing on minimising possible systematic effects in the analysis, with the aim of finding possible differences/similarities between the populations. Abundances are determined for Zr, La, Ce and Eu in 45 bulge giants and 291 local disk giants, from high-resolution optical spectra. The abundances are determined by fitting synthetic spectra using the SME-code. The disk sample is separated into thin/thick disk components using a combination of abundances and kinematics. We find flat Zr, La, Ce trends in the bulge, with a $\sim 0.1$ dex higher La abundance compared with the disk, possibly indicating a higher s-process contribution for La in the bulge. [Eu/Fe] decreases with increasing [Fe/H], with a plateau at around [Fe/H] $\sim -0.4$, pointing at similar enrichment as $\alpha$-elements in all populations. We find that the r-process dominated the neutron-capture production at early times both in the disks and bulge. [La/Eu] for the bulge are systematically higher than the thick disk, pointing to either a) a different amount of SN II or b) a different contribution of the s-process in the two populations. Considering [(La+Ce)/Zr], the bulge and the thick disk follow each other closely, suggesting a similar ratio of high/low mass asymptotic giant branch-stars.