Oxygen trends in the Galactic thin and thick disks

Abstract

We present oxygen abundances for 72 F and G dwarf stars in the solar neighbourhood. Using the kinematics of the stars we divide them into two sub-samples with space velocities that are typical for the thick and thin disks, respectively. The metallicities of the stars range from [Fe/H] ≈ -0.9 to +0.4 and we use the derived oxygen abundances of the stars to: (1) perform a differential study of the oxygen trends in the thin and the thick disk; (2) to follow the trend of oxygen in the thin disk to the highest metallicities. We analyze the forbidden oxygen lines at 6300 Å and 6363 Å as well as the (NLTE afflicted) triplet lines around 7774 Å. For the forbidden line at 6300 Å we have spectra of very high S/N (>400) and resolution (R ≳ 215 000). This has enabled a very accurate modeling of the oxygen line and the blending Ni lines. The high internal accuracy in our determination of the oxygen abundances from this line is reflected in the very tight trends we find for oxygen relative to iron. From these abundances we are able to draw the following major conclusions: (i) That the [O/Fe] trend at super-solar [Fe/H] continues downward which is in concordance with models of Galactic chemical evolution. This is not seen in previous studies as it has not been possible to take the blending Ni lines in the forbidden oxygen line at 6300 Å properly into account; (ii) That the oxygen trends in the thin and the thick disks are distinctly different. This confirms and extends previous studies of the other α-elements; (iii) That oxygen does not follow Mg at super-solar metallicities; (iv) We also provide an empirical NLTE correction for the infrared O I triplet that could be used for dwarf star spectra with a S/N such that only the triplet lines can be analyzed well, e.g. stars at large distances; (v) Finally, we find that Gratton et al. (1999) overestimate the NLTE corrections for the permitted oxygen triplet lines at ∼7774 Å for the parameter space that our stars span.