Aims. The aim of this paper is to explore and map the age and abundance structure of the stars in the nearby Galactic disk. Methods. We have conducted a high-resolution spectroscopic study of 714 F and G dwarf and subgiant stars in the Solar neighbourhood. The star sample has been kinematically selected to trace the Galactic thin and thick disks to their extremes, the metal-rich stellar halo, sub-structures in velocity space such as the Hercules stream and the Arcturus moving group, as well as stars that cannot (kinematically) be associated with either the thin disk or the thick disk. The determination of stellar parameters and elemental abundances is based on a standard 1-D LTE analysis using equivalent width measurements in high-resolution (R = 40 000 − 110 000) and high signal-to-noise (S /N = 150 − 300) spectra obtained with FEROS on the ESO 1.5-m and 2.2-m telescopes, SOFIN and FIES on the Nordic Optical Telescope, UVES on the ESO Very Large Telescope, HARPS on the ESO 3.6-m telescope, and MIKE on the Magellan Clay telescope. NLTE corrections for individual Fe i lines were employed in every step of the analysis. Results. We present stellar parameters, stellar ages, kinematical parameters, orbital parameters, and detailed elemental abundances for O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, and Ba for 714 nearby F and G dwarf stars. Our data show that there is an old and α-enhanced disk population, and a younger and less α-enhanced disk population. While they overlap greatly in metallicity between −0.7 < [Fe/H] +0.1, they show a bimodal distribution in [α/Fe]. This bimodality becomes even clearer if stars where stellar parameters and abundances show larger uncertainties (T eff 5400 K) are discarded, showing that it is important to constrain the data set to a narrow range in the stellar parameters if small differences between stellar populations are to be revealed. We furthermore find that the α-enhanced population has orbital parameters placing the stellar birthplaces in the inner Galactic disk while the low-α stars mainly come from the outer Galactic disk, fully consistent with the recent claims of a short scale-length for the α-enhanced Galactic thick disk. We have also investigated the properties of the Hercules stream and the Arcturus moving group and find that neither of them present chemical or age signatures that could point to that they are disrupted clusters or extragalactic accretion remnants from ancient merger events. Instead, they are most likely dynamical features originating within the Galaxy. We furthermore have discovered that a standard 1-D, LTE analysis, utilising ionisation and excitation balance of Fe i and Fe ii lines produces a flat lower main sequence. As the exact cause for this effect is unclear we chose to apply an empirical correction. Turn-off, and more evolved, stars, appears to be un-affected.

Abstract. 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 [Fe/H] +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 α-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 ∼ 48 000 and S /N 150, respectively) recorded with the FEROS spectrograph on La Silla, Chile. Abundances have been determined for four α-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 ∼30 000 spectral lines. An extensive investigation of the atomic parameters, log g f -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  and Ti . Our solar Ti abundances are in concordance with the standard meteoritic Ti abundance.

We present a re-analysis of the Geneva-Copenhagen survey, which benefits from the infrared flux method to improve the accuracy of the derived stellar effective temperatures and uses the latter to build a consistent and improved metallicity scale. Metallicities are calibrated on high-resolution spectroscopy and checked against four open clusters and a moving group, showing excellent consistency. The new temperature and metallicity scales provide a better match to theoretical isochrones, which are used for a Bayesian analysis of stellar ages. With respect to previous analyses, our stars are on average 100 K hotter and 0.1 dex more metal rich, which shift the peak of the metallicity distribution function around the solar value. From Strömgren photometry we are able to derive for the first time a proxy for [α/Fe] abundances, which enables us to perform a tentative dissection of the chemical thin and thick disc. We find evidence for the latter being composed of an old, mildly but systematically alpha-enhanced population that extends to super solar metallicities, in agreement with spectroscopic studies. Our revision offers the largest existing kinematically unbiased sample of the solar neighbourhood that contains full information on kinematics, metallicities, and ages and thus provides better constraints on the physical processes relevant in the build-up of the Milky Way disc, enabling a better understanding of the Sun in a Galactic context.

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. , A&A, 410, 527, 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 α-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 [Fe/H] +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 250) and high resolution (R ∼ 80 000, 45 000, and 110 000 for the SOFIN, FEROS, and UVES spectra, respectively).