Elemental abundance trends in the Galactic thin and thick disks as traced by nearby F and G dwarf stars

Based on spectra from F and G dwarf stars, we present elemental abundance trends in the Galactic thin and thick disks in the metallicity regime - 0.8 less than or similar to [Fe/H] less than or similar to + 0.4. Our findings can be summarized as follows. 1) Both the thin and the thick disks show smooth and distinct abundance trends that, at sub-solar metallicities, are clearly separated. 2) For the alpha-elements the thick disk shows signatures of chemical enrichment from SNe type Ia. 3) The age of the thick disk sample is in the mean older than the thin disk sample. 4) Kinematically, there exist thick disk stars with super-solar metallicities. Based on these findings, together with other constraints from the literature, we discuss different formation scenarios for the thick disk. We suggest that the currently most likely formation scenario is a violent merger event or a close encounter with a companion galaxy. Based on kinematics the stellar sample was selected to contain stars with high probabilities of belonging either to the thin or to the thick Galactic disk. The total number of stars are 66 of which 21 belong to the thick disk and 45 to the thin disk. The analysis is based on high-resolution spectra with high signal-to-noise (R similar to 48 000 and S/N greater than or similar to 150, respectively) recorded with the FEROS spectrograph on La Silla, Chile. Abundances have been determined for four alpha-elements ( Mg, Si, Ca, and Ti), for four even-nuclei iron peak elements (Cr, Fe, Ni, and Zn), and for the light elements Na and Al, from equivalent width measurements of similar to 30 000 spectral lines. An extensive investigation of the atomic parameters, log gf-values in particular, have been performed in order to achieve abundances that are trustworthy. Noteworthy is that we find for Ti good agreement between the abundances from Ti I and Ti II. Our solar Ti abundances are in concordance with the standard meteoritic Ti abundance.