Issue |
A&A
Volume 372, Number 2, June III 2001
|
|
---|---|---|
Page(s) | 601 - 615 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20010483 | |
Published online | 15 June 2001 |
On OH line formation and oxygen abundances in metal-poor stars
Astronomiska Observatoriet, Box 515, SE-751 20 Uppsala, Sweden e-mail: martin@astro.uu.se, aegp@astro.uu.se
Corresponding author: M. Asplund, martin@astro.uu.se
Received:
18
December
2000
Accepted:
26
March
2001
The formation of the UV OH spectral lines
has been investigated for a range of stellar parameters
in the light of 3D hydrodynamical model atmospheres.
The low atmospheric temperatures encountered at low metallicities
compared with the radiative equilibrium values enforced in classical
1D hydrostatic model atmospheres have a profound impact on the OH
line strengths. As a consequence, the derived O abundances using 3D models
are found to be systematically lower by more than 0.6 dex at
[Fe/H] compared with previous 1D analyses,
casting doubts on the recent claims for a monotonic increase in [O/Fe] towards
lower metallicities.
In fact, taken at face value the resulting 3D LTE trend is in rough
agreement with the conventional [O/Fe] plateau.
Caution must, however, be exercised in view of the remaining assumptions
in the 3D calculations.
We have verified that the stellar parameters remain essentially
unchanged with 3D model atmospheres
provided that the infrared flux method (
K),
Hipparcos parallaxes (
) and Fe ii
lines (
dex) are utilised, leaving the
3D O abundances from OH lines largely intact
(
dex).
Greater concern stems from possible departures from LTE in
both the line formation and the molecular equilibrium, which, if present,
would increase the derived O abundances again.
Non-LTE line formation calculations with 1D model atmospheres
suggest no significant steepening of the [O/Fe] trend even if the
abundance corrections amount to about 0.2 dex for all
investigated stellar parameters. We note, however, that
the 3D case may not necessarily be as metallicity-independent.
The apparent lack of laboratory or theoretical
rate coefficients at the relevant temperatures for the involved
molecular reactions unfortunately
prevents a quantitative discussion on the possible
effects of non-equilibrium chemistry.
Key words: convection / line: formation / stars: abundances / stars: atmospheres / stars: population II / galaxy: evolution
© ESO, 2001
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