Abundances of neutron-capture elements in thin- and thick-disc stars in the solar neighbourhood

Abstract

The aim of this work is to determine abundances of neutron-capture elements for thin- and thick-disc F, G, and K stars in several sky fields near the north ecliptic pole and to compare the results with the Galactic chemical evolution models, to explore elemental gradients according to stellar ages, mean galactocentric distances, and maximum heights above the Galactic plane. The observational data were obtained with the 1.65m telescope at the Moletai Astronomical Observatory and a fibre-fed high-resolution spectrograph. Elemental abundances were determined using a differential spectrum synthesis with the MARCS stellar model atmospheres and accounting for the hyperfine-structure effects. We determined abundances of Sr, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, and Eu for 424 thin- and 82 thick-disc stars. The sample of thick-disc stars shows a clearly visible decrease in [Eu/Mg] with increasing [Fe/H] compared to the thin-disc stars, bringing more evidence of a different chemical evolution in these two Galactic components. Abundance correlation with age slopes for the investigated thin-disc stars are slightly negative for the majority of s-process dominated elements, while r-process dominated elements have positive correlations. Our sample of thin-disc stars with ages spanning from 0.1 to 9 Gyrs give the [Y/Mg]=0.022 ($\pm$0.015)-0.027 ($\pm$0.003)*age [Gyr] relation. For the thick-disc stars, when we also took data from other studies into account, we found that [Y/Mg] cannot serve as an age indicator. The radial [El/Fe] gradients in the thin disc are negligible for the s-process dominated elements and become positive for the r-process dominated elements. The vertical gradients are negative for the light s-process dominated elements and become positive for the r-process dominated elements. In the thick disc, the radial [El/Fe] slopes are negligible, and the vertical slopes are predominantly negative.

20 pages, 18 figures, accepted for publication in Astronomy & Astrophysics; clarified citation from one of the references

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