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

A&A 390, 235-251 (2002)
DOI: 10.1051/0004-6361:20020736

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

P. E. Nissen¹, F. Primas², M. Asplund^{3, 4} and D. L. Lambert⁵

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

(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 $\ion{O}{i}$ 7774 Å triplet, and a selection of weak $\ion{Fe}{ii}$ lines observed on high-resolution spectra acquired with the VLT UVES spectrograph. The line is detected in the spectra of 18 stars with $-2.4 < \mbox{\rm [Fe/H]}< -0.5$ , 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] $\simeq$ +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 $\ion{O}{i}$ 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 $\ion{Fe}{ii}$ 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 $\ion{Fe}{ii}$ lines (excitation effect). The [O/Fe] vs. [Fe/H] relation remains quasi-linear extending to [O/Fe] $\simeq$ +0.5 at [Fe/H] = -2.5, but with a tendency of a plateau with [O/Fe] $\simeq$ +0.3 for -2.0 < [Fe/H] < -1.0, and a hint of cosmic scatter in [O/Fe] at $\mbox{\rm [Fe/H]}\simeq -1.0$ . Use of the hydrodynamical models disturbs the broad agreement between the oxygen abundances from the , $\ion{O}{i}$ 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 $\ion{O}{i}$ lines.

Key words: stars: abundances -- stars: atmospheres -- stars: fundamental parameters -- Galaxy: evolution

Offprint request: P. E. Nissen, pen@ifa.au.dk

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