alpha-, r-, and s-process element trends in the Galactic thin and thick disks

Abstract

From a detailed elemental abundance analysis of 102 F and G dwarf stars we present abundance trends in the Galactic thin and thick disks for 14 elements (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba, and Eu). Stellar parameters and elemental abundances (except for Y, Ba and Eu) for 66 of the 102 stars were presented in our previous studies (Bensby et al. 2003, 2004a). The 36 stars that are new in this study extend and confirm our previous results and allow us to draw further conclusions regarding abundance trends. The s-process elements Y and Ba, and the r-element Eu have also been considered here for the whole sample for the first time. With this new larger sample we now have the following results: 1) Smooth and distinct trends that for the thin and thick disks are clearly separated; 2) The alpha-element trends for the thick disk show typical signatures from the enrichment of SNIa; 3) The thick disk stellar sample is in the mean older than the thin disk stellar sample; 4) The thick disk abundance trends are invariant with galactocentric radii (R_m); 5) The thick disk abundance trends appear to be invariant with vertical distance (Z_max) from the Galactic plane. Adding further evidence from the literaure we argue that a merger/interacting scenario with a companion galaxy to produce a kinematical heating of the stars (that make up today's thick disk) in a pre-existing old thin disk is the most likely formation scenario for the Galactic thick disk.