Issue |
A&A
Volume 500, Number 3, June IV 2009
|
|
---|---|---|
Page(s) | 1221 - 1238 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/200809640 | |
Published online | 16 April 2009 |
Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity
1
Research School of Astronomy & Astrophysics, The Australian National University, Mount Stromlo Observatory, Cotter Road, Weston ACT 2611, Australia e-mail: damian@iac.es
2
Max Planck Institute for Astrophysics, Postfach 1317, 85741 Garching b. München, Germany
3
Department of Physics and Astronomy, Uppsala University, Box 515, 751-20 Uppsala, Sweden
4
Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
5
The Institute for Solar Physics of the Royal Swedish Academy of Sciences, AlbaNova University Centre, 106 91 Stockholm, Sweden
Received:
24
February
2008
Accepted:
17
February
2009
Aims. A detailed study is presented, including estimates of the impact on elemental abundance analysis, of the non-local thermodynamic equilibrium (non-LTE) formation of the high-excitation neutral oxygen 777 nm triplet in model atmospheres representative of stars with spectral types F to K.
Methods. We have applied the statistical equilibrium code MULTI to
a number of plane-parallel MARCS atmospheric models
covering late-type stars (4500 ≤ Teff ≤ 6500 K, 2 ≤ log ≤ 5 [cgs], and
-3.5 ≤ [Fe/H] ≤ 0). The atomic
model employed includes, in
particular, recent quantum-mechanical electron collision data.
Results. We confirm that the O i triplet lines form
under non-LTE conditions in late-type
stars, suffering negative abundance corrections with
respect to LTE. At solar metallicity, the non-LTE effect,
mainly attributed in previous studies to photon losses in the triplet
itself, is also driven by an additional significant contribution from line
opacity. At low metallicity, the very pronounced departures from LTE are
due to overpopulation of the lower level (3s 5So) of the
transition. Large line opacity stems from triplet-quintet intersystem
electron collisions, a form of coupling previously not considered or
seriously underestimated. The non-LTE effects generally become
severe for models (both giants
and dwarfs) with higher Teff.
Interestingly, in metal-poor
turn-off stars, the negative non-LTE abundance corrections
tend to
rapidly become more severe towards lower metallicity. When neglecting
H collisions, they amount to as much as
~ 0.9 dex and ~1.2 dex, respectively at [Fe/H] = -3
and [Fe/H] = -3.5. Even when such collisions are included, the LTE abundance remains a serious
overestimate, correspondingly by
~ 0.5 dex and ~0.9 dex at such low metallicities. Although the
poorly known inelastic hydrogen collisions thus remain an important
uncertainty, the large metallicity-dependent non-LTE effects
seem to point to a resulting “low” (compared to LTE) [O/Fe] in metal-poor halo stars.
Conclusions. Our results may be important in solving the
long-standing [O/Fe] debate. When applying the derived non-LTE
corrections, the LTE oxygen abundance inferred from the
777 nm permitted triplet will be decreased
substantially at low metallicity. If the classical Drawin formula
is employed for O+H collisions, the derived [O/Fe]
trend becomes almost flat below [Fe/H] ~ -1, in better agreement
with recent literature estimates generally obtained from other
oxygen abundance indicators. A value of [O/Fe] +0.5 may therefore be appropriate, as
suggested by standard theoretical models of type II supernovae
nucleosynthetic yields.
If neglecting impacts with H atoms instead, [O/Fe] decreases towards lower [Fe/H],
which would open new questions. Our tests using ATLAS model atmospheres show that, though non-LTE
corrections for metal-poor dwarfs are smaller (by ~0.2 dex when adopting
efficient H collisions) than in the MARCS case, our main conclusions are preserved, and that the
LTE approach tends to seriously overestimate the O abundance at low metallicity.
However, in order to finally reach consistency between oxygen abundances
from the different available spectral features, it is of high priority to reduce the large uncertainty
regarding H collisions, to undertake a full investigation of the interplay of non-LTE and 3D
effects, and to clarify the issue of the temperature scale at low
metallicity.
Key words: line: formation / stars: abundances / stars: late-type / Galaxy: evolution
© ESO, 2009
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