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Three-dimensional hydrodynamical simulations of surface convection in red giant stars: Impact on spectral line formation and abundance analysis
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Astronomy and Space Physics.
2007 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 469, no 2, 687-706 p.Article in journal (Refereed) Published
Abstract [en]

Aims. We investigate the impact of realistic three-dimensional (3D) hydrodynamical model atmospheres of red giant stars at different metallicities on the formation of spectral lines of a number of ions and molecules.

Methods. We carry out realistic, ab initio, 3D, hydrodynamical simulations of surface convection at the surface of red giant stars with varying effective temperatures and metallicities. We use the convection simulations as time-dependent hydrodynamical model stellar atmospheres to calculate spectral lines of a number of ions (Li I, O I, Na I, Mg I, Ca I, Fe I, and Fe II) and molecules ( CH, NH, and OH) under the assumption of local thermodynamic equilibrium (LTE). We carry out a differential comparison of the line strengths computed in 3D with the results of analogous line formation calculations for classical, 1D, hydrostatic, plane-parallel marcs model atmospheres in order to estimate the impact of 3D models on the derivation of elemental abundances.

Results. The temperature and density inhomogeneities and correlated velocity fields in 3D models, as well as the differences between the mean 3D stratifications and corresponding 1D model atmospheres significantly affect the predicted strengths of spectral lines. Under the assumption of LTE, the low atmospheric temperatures encountered in 3D model atmospheres of very metal-poor giant stars cause spectral lines from neutral species and molecules to appear stronger than within the framework of 1D models. As a consequence, elemental abundances derived from these lines using 3D models are significantly lower than according to 1D analyses. In particular, the differences between 3D and 1D abundances of C, N, and O derived from CH, NH, and OH weak low-excitation lines are found to be in the range - 0.5 dex to - 1.0 dex for the the red giant stars at [Fe/H] = - 3 considered here. At this metallicity, large negative corrections ( about - 0.8 dex) are also found, in LTE, for weak low-excitation Fe I lines. We caution, however, that the neglected departures from LTE might be significant for these and other elements and comparable to the effects due to stellar granulation.

Place, publisher, year, edition, pages
2007. Vol. 469, no 2, 687-706 p.
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-94811DOI: 10.1051/0004-6361:20066321ISI: 000247671900025OAI: oai:DiVA.org:uu-94811DiVA: diva2:168799
Available from: 2006-09-01 Created: 2006-09-01 Last updated: 2017-12-14Bibliographically approved
In thesis
1. On the Chemical Composition of Metal-Poor Stars: Impact of Stellar Granulation and Departures from Local Thermodynamic Equilibrium on the Formation of Spectral Lines
Open this publication in new window or tab >>On the Chemical Composition of Metal-Poor Stars: Impact of Stellar Granulation and Departures from Local Thermodynamic Equilibrium on the Formation of Spectral Lines
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The information about the chemical compositions of stars is encoded in their spectra. Accurate determinations of these compositions are crucial for our understanding of stellar nucleosynthesis and Galactic chemical evolution. The determination of elemental abundances in stars requires models for the stellar atmospheres and the processes of line formation. Nearly all spectroscopic analyses of late-type stars carried out today are based on one-dimensional (1D), hydrostatic model atmospheres and on the assumption of local thermodynamic equilibrium (LTE). This approach can lead to large systematic errors in the predicted stellar atmospheric structures and line-strengths, and, hence, in the derived stellar abundances. In this thesis, examples of departures from LTE and from hydrostatic equilibrium are explored. The effects of background line opacities (line-blocking) due to atomic lines on the statistical equilibrium of Fe are investigated in late-type stars. Accounting for this line opacity is important at solar metallicity, where line-blocking significantly reduces the rates of radiatively induced ionizations of Fe. On the contrary, the effects of line-blocking in metal-poor stars are insignificant. In metal-poor stars, the dominant uncertainty in the statistical equilibrium of Fe is the treatment of inelastic H+Fe collisions. Substantial departures of Fe abundances from LTE are found at low metallicities: about 0.3 dex with efficient H+Fe collisions and about 0.5 dex without. The impact of three-dimensional (3D) hydrodynamical model atmospheres on line formation in red giant stars is also investigated. Inhomogeneities and correlated velocity fields in 3D models and differences between the mean 3D stratifications and corresponding 1D model atmospheres can significantly affect the predicted line strengths and derived abundances, in particular at very low metallicities. In LTE, the differences between 3D and 1D abundances of C, N, and O derived from CH, NH, and OH weak low-excitation lines are in the range -0.5 dex to -1.0 dex at [Fe/H]=-3. Large negative corrections (about -0.8 dex) are also found in LTE for weak low-excitation neutral Fe lines. We also investigate the impact of 3D hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper iron-poor stars HE 0107-5240 and HE 1327-2326. The lower temperatures of the line-forming regions of the 3D models compared with 1D models cause changes in the predicted spectral line strengths. In particular we find the 3D abundances of C, N, and O to be lower by about -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundance of Fe is decreased but only by -0.2 dex. Departures from LTE for Fe might actually be very large for these stars and dominate over the effects due to granulation.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2006. 71 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 214
Keyword
Astronomy, late-type stars, stellar abundances, stellar atmospheres, spectral line formation, convection, hydrodynamics, Galactic chemical evolution, Astronomi
Identifiers
urn:nbn:se:uu:diva-7121 (URN)91-554-6641-9 (ISBN)
Public defence
2006-09-22, Polhemsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 13:15
Opponent
Supervisors
Available from: 2006-09-01 Created: 2006-09-01Bibliographically approved

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