Mg, Ba and Eu abundances in thick disk and halo stars

Abstract

Our sample of cool dwarf stars from previous papers is extended in this study including 15 moderately metal-deficient stars. The samples of halo and thick disk stars have overlapping metallicities with [Fe/H] in the region from -0.9 to -1.5, and we first compare chemical properties of these two kinematically different stellar populations independent on their metallicity. We present barium, europium and magnesium abundances for the new sample of stars. The results are based on NLTE line formation obtained in differential model atmosphere analyses of high resolution spectra observed mainly using the UVES spectrogragh at the VLT of ESO. We confirm the overabundance of Eu relative to Mg in the halo stars. Eight halo stars show the [Eu/Mg] ratios between 0.23 and 0.41, whereas stars in the thick and thin disk display a solar Eu to Mg ratio. The [Eu/Ba] values found in the thick disk stars to lie between 0.35 and 0.57 suggest that during the thick disk formation evolved low-mass stars started to enrich the interstellar gas by s-nuclei of Ba, and the s-process contribution to Ba varies from 30% to 50%. Based on these results, and using the chemical evolution calculations by Travaglio et al. (1999), we estimate that the thick disk stellar population formed on a timescale from 1.1 to 1.6 Gyr from the beginning of the protogalactic collapse. In the halo stars the [Eu/Ba] ratios are found mostly between 0.40 and 0.67, which suggests a duration of the halo formation of about 1.5 Gyr. For the whole sample of stars we present the even-to-odd Ba isotope ratios as determined from hyperfine structure seen in the Ba II resonance line lambda4554. Based on these data we deduce for the thick disk stars the ratio of the s/r-process contribution to barium as 30 : 70 (+- 30%), in agreement with the results obtained from the [Eu/Ba] ratios.

Accepted for A&A, 10 pages, 6 figures

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