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 abundance 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 SN Ia ; 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 . The 102 stars have -1 \lesssim [ Fe / H ] \lesssim + 0.4 and are all in the solar neighbourhood .
Based on their kinematics they have been divided into a thin disk sample and a thick disk sample consisting of 60 and 38 stars , respectively .
The remaining 4 stars have kinematics that make them kinematically intermediate to the two disks .
Their chemical abundances also place them in between the two disks .
Which of the two disk populations these 4 stars belong to , or if they form a distinct population of their own , can at the moment not be settled .
The 66 stars from our previous studies were observed with the FEROS spectrograph on the ESO 1.5-m telescope and the CES spectrograph on the ESO 3.6-m telescope .
Of the 36 new stars presented here 30 were observed with the SOFIN spectrograph on the Nordic Optical Telescope on La Palma , 3 with the UVES spectrograph on VLT/UT2 , and 3 with the FEROS spectrograph on the ESO 1.5-m telescope .
All spectra have high signal-to-noise ratios ( typically S / N \gtrsim 250 ) and high resolution ( R \sim 80 000 , 45 000 , and 110 000 for the SOFIN , FEROS , and UVES spectra , respectively ) .