Galactic Evolution of Oxygen: OH lines in 3D hydrodynamical model atmospheres

Abstract

The measurement of oxygen lines in metal-poor unevolved stars, in particular near-UV OH lines, can provide invaluable information on the properties of the Early Galaxy. Near-UV OH lines constitute an important tool to derive oxygen abundances in metal-poor dwarf stars. Therefore, it is important to correctly model the line formation of OH lines, especially in metal-poor stars, where 3D hydrodynamical models commonly predict cooler temperatures than plane-parallel hydrostatic models in the upper photosphere. We have made use of a grid of 52 3D hydrodynamical model atmospheres for dwarf stars computed using the code CO5BOLD, extracted from the more extended CIFIST grid. The 52 models cover the effective temperature range 5000-6500K, the surface gravity range 3.5-4.5 and the metallicity range -3<[Fe/H]<0. We determine 3D-LTE abundance corrections in all the 52 3D models for several OH lines and FeI lines of different excitation potentials. These 3D-LTE corrections are generally negative reaching values of roughly -1 dex (for the OH 3167 with excitation potential of approximately 1 eV) for the higher temperatures and surface gravities. We apply these 3D-LTE corrections to the individual O abundances, derived from OH lines, of a sample the metal-poor dwarf stars reported in Israelian et al.(1998, 2001) and Boesgaard et al.(1999), by interpolating the stellar parameters of the dwarfs in the grid of 3D-LTE corrections. The new 3D-LTE [O/Fe] ratio still keeps a similar trend as the 1D-LTE, i.e, increasing towards lower [Fe/H] values. We applied 1D-NLTE corrections to 3D FeI abundances and we still see an increasing [O/Fe] ratio towards lower metallicites. However, the Galactic [O/Fe] ratio must be revisited once 3D-NLTE corrections become available for OH and Fe lines for a grid of 3D hydrodynamical model atmospheres.