Volume 390, Number 1, July IV 2002
|Page(s)||235 - 251|
|Section||Stellar structure and evolution|
|Published online||05 July 2002|
O/Fe in metal-poor main sequence and subgiant stars*
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: email@example.com
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: firstname.lastname@example.org
5 Department of Astronomy, University of Texas, Austin, TX 78712–1083, USA e-mail: email@example.com
Corresponding author: P. E. Nissen, firstname.lastname@example.org
Accepted: 15 May 2002
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.
Key words: stars: abundances / stars: atmospheres / stars: fundamental parameters / Galaxy: evolution
© ESO, 2002
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