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Oxygen abundances in metal-poor subgiants as determined from [OI], OI and OH lines
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics, The Uppsala Astronomical Observatory.
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2006 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 451, no 2, 621-642 p.Article in journal (Refereed) Published
Abstract [en]

The debate on the oxygen abundances of metal-poor stars has its origin in contradictory results obtained using different abundance indicators. To achieve a better understanding of the problem we have acquired high quality spectra with the Ultraviolet and Visual Echelle Spectrograph at VLT, with a signal-to-noise of the order of 100 in the near ultraviolet and 500 in the optical and near infrared wavelength range. Three different oxygen abundance indicators, OH ultraviolet lines around 310.0 nm, the [O I] line at 630.03 nm and the O I lines at 777.1-5 nm were observed in the spectra of 13 metal-poor subgiants with . Oxygen abundances were obtained from the analysis of these indicators which was carried out assuming local thermodynamic equilibrium and plane-parallel model atmospheres. Abundances derived from O I were corrected for departures from local thermodynamic equilibrium. Stellar parameters were computed using -vs.-color calibrations based on the infrared flux method and Balmer line profiles, Hipparcos parallaxes and lines. [O/Fe] values derived from the forbidden line at 630.03 nm are consistent with an oxygen/iron ratio that varies linearly with [Fe/H] as . Values based on the O I triplet are on average (s.d.) higher than the values based on the forbidden line while the agreement between OH ultraviolet lines and the forbidden line is much better with a mean difference of the order of (s.d.). In general, our results follow the same trend as previously published results with the exception of the ones based on OH ultraviolet lines. In that case our results lie below the values which gave rise to the oxygen abundance debate for metal-poor stars.

Place, publisher, year, edition, pages
2006. Vol. 451, no 2, 621-642 p.
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-92684DOI: 10.1051/0004-6361:20053181OAI: oai:DiVA.org:uu-92684DiVA: diva2:165854
Available from: 2005-02-24 Created: 2005-02-24 Last updated: 2014-01-28Bibliographically approved
In thesis
1. On the Abundances of Li, Be and O in Metal-Poor Stars in the Galaxy
Open this publication in new window or tab >>On the Abundances of Li, Be and O in Metal-Poor Stars in the Galaxy
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Stellar atmospheres constitute excellent environments to study the chemical evolution of our Galaxy. The chemical composition of these atmospheres reflects the composition of the gas from where these stars were born. As the Galaxy evolves, the composition of the gas changes from being primordial (Big-Bang nucleosynthesis) to being enriched in heavy elements (stellar and interstellar nucleosynthesis). The abundances of fragile chemical elements can be affected by stellar mixing processes. Precise lithium, beryllium and oxygen abundance determinations in old stars are presented in this thesis. These determinations are based on the analysis of the observed spectra of a sample of thirteen metal-poor subgiant stars. According to stellar mixing theories, these stars are in a stellar evolutionary stage in which mixing by convection is expected. Abundances of fragile elements like lithium and beryllium are thus expected to be affected by such mixing processes. As a consequence of this, the abundances of these elements are discussed in a dilution context. Lithium and beryllium abundances are compared with the abundances of stars with similar characteristics but in a less evolved stellar phase so that mixing processes have not acted yet. As expected, our abundances seem to be depleted following reasonably well the standard predictions. Stellar abundances of oxygen should give an estimate of the oxygen contribution of core-collapse supernovae to the interstellar medium. However, there is poor agreement among the abundances determined from different atomic or molecular indicators in general. Abundances coming from three different indicators are compared in this thesis. The abundances determined from the O I infrared triplet lines at 777.1-5 nm give the poorest agreement among the three indicators. The abundances based on OH ultraviolet lines around 310 nm are lower for the subgiants in comparison with previous studies of main-sequence stars, becoming even lower than values based on the O I forbidden line at 630.03 nm. Still the most reliable indicator appears to be the O I forbidden line which suggests a plateau-like or only slowly increasing [O/Fe] towards lower [Fe/H]. In addition, the line formation of the Be II ultraviolet resonance lines at 313.0-1 nm, commonly used for abundance determinations purposes, is investigated under non-local thermodynamic equilibrium. We find that the common assumption of local thermodynamic equilibrium typically gives systematic errors of about 0.1 dex.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2005. vii + 45 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 20
Astronomy, galactic evolution, stellar abundances, stellar atmospheres, spectral line formation, stellar mixing, Astronomi
National Category
Astronomy, Astrophysics and Cosmology
urn:nbn:se:uu:diva-4814 (URN)91-554-6163-8 (ISBN)
Public defence
2005-03-18, Polhemsalen, Ångström Laboratory, Uppsala, 13:15
Available from: 2005-02-24 Created: 2005-02-24Bibliographically approved

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