Galactic evolution of oxygen

OH lines in 3D hydrodynamical model atmospheres

¹ Dpto. de Astrofísica y Ciencias de la Atmósfera, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain e-mail: jonay@astrax.fis.ucm.es
² Cosmological Impact of the First STars (CIFIST) Marie Curie Excellence Team , France
³ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁴ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁵ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
⁶ Institut d'Astronomie et d'Astrophysique, Université Libre de Bruxelles, 1050 Bruxelles, Belgium
⁷ CRAL,UMR 5574: CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 allée d'Italie, 69364 Lyon Cedex 7, France
⁸ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napels, Italy

Received: 10 March 2010
Accepted: 17 May 2010

Abstract

Context. Oxygen is the third most common element in the Universe. The measurement of oxygen lines in metal-poor unevolved stars, in particular near-UV OH lines, can provide invaluable information about the properties of the Early Galaxy.

Aims. 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.

Methods. We have made use of a grid of 52 3D hydrodynamical model atmospheres for dwarf stars computed with the code CO⁵BOLD, extracted from the more extended CIFIST grid. The 52 models cover the effective temperature range 5000–6500 K, the surface gravity range 3.5–4.5 and the metallicity range -3 < [Fe/H] < 0.

Results. We determine 3D-LTE abundance corrections in all 52 3D models for several OH lines and  lines of different excitation potentials. These 3D-LTE corrections are generally negative and reach values of roughly –1 dex (for the OH 3167 with excitation potential of approximately 1 eV) for the higher temperatures and surface gravities.

Conclusions. 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, ApJ, 507, 805), Israelian et al. (2001, ApJ, 551, 833) and Boesgaard et al. (1999, AJ, 117, 492) 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 abundances and 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.