2007A&A...465..271R

The abundances of iron and oxygen are homogeneously determined in a sample of 523 nearby (d<150pc) FGK disk and halo stars with metallicities in the range -1.5<[Fe/H]<0.5. Iron abundances were obtained from an LTE analysis of a large set of FeI and FeII lines with reliable atomic data. Oxygen abundances were inferred from a restricted non-LTE analysis of the 777nm OI triplet. We adopted the infrared flux method temperature scale and surface gravities based on Hipparcos trigonometric parallaxes. Within this framework, the ionization balance of iron lines is not satisfied: the mean abundances from the FeI lines are systematically lower by 0.06dex than those from the FeII lines for dwarf stars of T_eff>5500K and [Fe/H]<0.0, and giant stars of all temperatures and metallicities covered by our sample. The discrepancy worsens for cooler and metal-rich main-sequence stars. We use the stellar kinematics to compute the probabilities of our sample stars to be members of the thin disk, thick disk, or halo of the Galaxy. We find that the majority of the kinematically-selected thick-disk stars show larger [O/Fe] ratios compared to thin-disk stars while the rest show thin-disk abundances, which suggests that the latter are thin-disk members with unusual (hotter) kinematics. A close examination of this pattern for disk stars with ambiguous probabilities shows that an intermediate population with properties between those of the thin and thick disks does not exist, at least in the solar neighborhood. Excluding the stars with unusual kinematics, we find that thick-disk stars show slowly decreasing [O/Fe] ratios from about 0.5 to 0.4 in the -0.8<[Fe/H]←0.3 range. Using a simple model for the chemical evolution of the thick disk we show that this trend results directly from the metallicity dependence of the Type II supernova yields. At [Fe/H]>-0.3, we find no obvious indication of a sudden decrease (i.e., a ``knee'') in the [O/Fe] vs. [Fe/H] pattern of thick-disk stars that would connect the thick and thin disk trends at a high metallicity. We conclude that Type Ia supernovae (SN Ia) did not contribute significantly to the chemical enrichment of the thick disk. In the -0.8<[Fe/H]<+0.3 range, thin-disk stars show decreasing [O/Fe] ratios from about 0.4 to 0.0 that require a SN Ia contribution. The implications of these results for studies of the formation and evolution of the Galactic disk are discussed.