The abundances of iron and oxygen are homogeneously determined in a sample of 523 nearby ( d < 150 pc ) FGK disk and halo stars with metallicities in the range -1.5 < \mathrm { [ Fe / H ] } < 0.5 .
Iron abundances were obtained from an LTE analysis of a large set of Fe i and Fe ii lines with reliable atomic data .
Oxygen abundances were inferred from a restricted non-LTE analysis of the 777 nm O i 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 Fe i lines are systematically lower by 0.06 dex than those from the Fe ii lines for dwarf stars of T _ { \mathrm { eff } } > 5500 K and \mathrm { [ 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 \mathrm { [ O / Fe ] } ratios from about 0.5 to 0.4 in the -0.8 < \mathrm { [ 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 \mathrm { [ Fe / H ] } > -0.3 , we find no obvious indication of a sudden decrease ( i.e. , a ‘ knee ’ ) in the \mathrm { [ O / Fe ] } vs . \mathrm { [ 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 < \mathrm { [ Fe / H ] } < +0.3 range , thin-disk stars show decreasing \mathrm { [ 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 .