O/Fe in metal-poor main sequence and subgiant stars^*

¹ Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark
² European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching b. München, Germany e-mail: fprimas@eso.org
³ Uppsala Astronomical Observatory, Box 515, 75120, Sweden
⁴ Present address: Research School of Astronomy and Astrophysics, Australian National University, Mount Stromlo Observatory, Cotter Road, Weston, ACT 2611, Australia e-mail: martin@mso.anu.edu.au
⁵ Department of Astronomy, University of Texas, Austin, TX 78712–1083, USA e-mail: dll@anchor.as.utexas.edu

Corresponding author: P. E. Nissen, pen@ifa.au.dk

Received: 17 April 2002
Accepted: 15 May 2002

Abstract

A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is presented using the  6300 Å line, the  7774 Å triplet, and a selection of weak  lines observed on high-resolution spectra acquired with the VLT UVES spectrograph. The  line is detected in the spectra of 18 stars with , and the triplet is observed for 15 stars with  ranging from -1.0 to -2.7. The abundance analysis was made first using standard model atmospheres taking into account non-LTE effects on the triplet: the  line and the triplet give consistent results with [O/Fe] increasing quasi-linearly with decreasing [Fe/H] reaching [O/Fe] +0.7 at [Fe/H] = -2.5. This trend is in reasonable agreement with other results for [O/Fe] in metal-poor dwarfs obtained using standard atmospheres and both ultraviolet and infrared OH lines. There is also broad agreement with published results for [O/Fe] for giants obtained using standard model atmospheres and the line, and the OH infrared lines, but the  lines give higher [O/Fe] values which may, however, fall into place when non-LTE effects are considered. When hydrodynamical model atmospheres representing stellar granulation in dwarf and subgiant stars replace standard models, the [O/Fe] from the  and  lines is decreased by an amount which increases with decreasing [Fe/H]. These 3D effects on [O/Fe] is compounded by the opposite behaviour of the  (continuous opacity effect) and  lines (excitation effect). The [O/Fe] vs. [Fe/H] relation remains quasi-linear extending to [O/Fe] +0.5 at [Fe/H] = -2.5, but with a tendency of a plateau with [O/Fe] +0.3 for [Fe/H] , and a hint of cosmic scatter in [O/Fe] at . Use of the hydrodynamical models disturbs the broad agreement between the oxygen abundances from the ,   and OH lines, but 3D non-LTE effects may serve to erase these differences. The [O/Fe] values from the  line and the hydrodynamical model atmospheres for dwarfs and subgiant stars are lower than the values for giants using standard model atmospheres and the   and  lines.