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Atomic Processes in Stellar Atmospheres: Inelastic Collisions and Effects on Late-type Spectra
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Astrophysics.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Chemical abundances as measured from stellar spectral lines are often subject to uncertainties due to lack of accurate data for inelastic collisions, which is needed for non-local thermodynamic equilibrium (non-LTE) modelling. For cool stars, understanding of collision processes with electrons and hydrogen atoms is required to achieve high precision measurements. In this thesis, I have investigated the role of these collisions on the non-LTE formation of Li and Mg spectral lines in late-type stars.

In the case of Li, electron impact excitation processes were calculated using the R-matrix with pseudo states method and the results found to agree well with recent calculations using the convergent close-coupling technique. These modern data were employed in non-LTE calculations by updating an existing model atom, which already included modern data for hydrogen collision processes. Our results were compared with calculations using older semi-empirical approximation calculations and only small differences were found: about 0.01 dex (~ 2%) or less in the abundance corrections. We therefore conclude that the influence of uncertainties in the electron collision data on non-LTE calculations is negligible. Indeed, together with the collision data for the charge transfer process Li + H ↔ Li+ + H- now available, and barring the existence of an unknown important collisional process, the collisional data in general is not a source of significant uncertainty in non-LTE Li line formation calculations.

In the case of Mg, electron impact excitation processes were again calculated with the Rmatrix with pseudo states method, and used together with recent hydrogen collision calculations to build and test a model atom, without free parameters, for non-LTE modelling. Both electron and hydrogen collision processes, including charge transfer and excitation, are found to be important thermalising agents in various cases. The modelled spectra agree well with observed spectra from benchmark stars in the optical and infrared. The modelling predicts non-LTE abundance corrections ∆A(Mg)NLTE–LTE in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex. Results of calculations in a large grid of 1D model atmospheres are presented, and the implications for studies of Mg discussed. The propagation of uncertainties in the inelastic collision data to those in stellar abundances is investigated, and found to lead to small uncertainties, once again typically less than 0.01 dex (2%), although for few stellar models in specific lines (e.g., metal-poor suns, in the 7691 Å line) uncertainties can be as large as 0.03 dex (7%).

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2015. , 87 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1249
Keyword [en]
Spectral line: formation, atomic data, stellar abundances.
National Category
Astronomy, Astrophysics and Cosmology Atom and Molecular Physics and Optics
Research subject
Physics with specialization in Astrophysics
Identifiers
URN: urn:nbn:se:uu:diva-249168ISBN: 978-91-554-9240-3 (print)OAI: oai:DiVA.org:uu-249168DiVA: diva2:802180
Public defence
2015-06-03, Polhemssalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2015-05-13 Created: 2015-04-11 Last updated: 2015-07-07
List of papers
1. The influence of electron collisions on non-LTE Li line formation in stellar atmospheres
Open this publication in new window or tab >>The influence of electron collisions on non-LTE Li line formation in stellar atmospheres
2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 529, A31- p.Article in journal (Refereed) Published
Place, publisher, year, edition, pages
EDP Sciences, 2011
Keyword
atomic data, line: formation, stars: abundances
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-152922 (URN)10.1051/0004-6361/201016418 (DOI)000289557200039 ()
Available from: 2011-05-03 Created: 2011-05-03 Last updated: 2017-12-11
2. Mg line formation in late-type stellar atmospheres: I. The model atom
Open this publication in new window or tab >>Mg line formation in late-type stellar atmospheres: I. The model atom
Show others...
2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 579, A53Article in journal (Refereed) Published
Abstract [en]

Context. Magnesium is an element of significant astrophysical importance, often traced in late-type stars using lines of neutral magnesium, which is expected to be subject to departures from local thermodynamic equilibrium (LTE). The importance of Mg, together with the unique range of spectral features in late-type stars probing different parts of the atom, as well as its relative simplicity from an atomic physics point of view, makes it a prime target and test bed for detailed ab initio non-LTE modelling in stellar atmospheres. Previous non-LTE modelling of spectral line formation has, however, been subject to uncertainties due to lack of accurate data for inelastic collisions with electrons and hydrogen atoms.

Aims. In this paper we build and test a Mg model atom for spectral line formation in late-type stars with new or recent inelastic collision data and no associated free parameters. We aim to reduce these uncertainties and thereby improve the accuracy of Mg non-LTE modelling in late-type stars.

Methods. For the low-lying states of Mg I, electron collision data were calculated using the R-matrix method. Hydrogen collision data, including charge transfer processes, were taken from recent calculations by some of us. Calculations for collisional broadening by neutral hydrogen were also performed where data were missing. These calculations, together with data from the literature, were used to build a model atom. This model was then employed in the context of standard non-LTE modelling in 1D (including average 3D) model atmospheres in a small set of stellar atmosphere models. First, the modelling was tested by comparisons with observed spectra of benchmark stars with well-known parameters. Second, the spectral line behaviour and uncertainties were explored by extensive experiments in which sets of collisional data were changed or removed.

Results. The modelled spectra agree well with observed spectra from benchmark stars, showing much better agreement with line profile shapes than with LTE modelling. The line-to-line scatter in the derived abundances shows some improvements compared to LTE (where the cores of strong lines must often be ignored), particularly when coupled with averaged 3D models. The observed Mg emission features at 7 and 12 mu m in the spectra of the Sun and Arcturus, which are sensitive to the collision data, are reasonably well reproduced. Charge transfer with H is generally important as a thermalising mechanism in dwarfs, but less so in giants. Excitation due to collisions with H is found to be quite important in both giants and dwarfs. The R-matrix calculations for electron collisions also lead to significant differences compared to when approximate formulas are employed. The modelling predicts non-LTE abundance corrections Delta A(Mg)(NLTE-LTE) in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex.

Conclusions. Our results emphasise the need for accurate data of Mg collisions with both electrons and H atoms for precise non-LTE predictions of stellar spectra, but demonstrate that such data can be calculated and that ab initio non-LTE modelling without resort to free parameters is possible. In contrast to Li and Na, where only the introduction of charge transfer processes has led to differences with respect to earlier non-LTE modelling, the more complex case of Mg finds changes due to improvements in the data for collisional excitation by electrons and hydrogen atoms, as well as due to the charge transfer processes. Grids of departure coefficients and abundance corrections for a range of stellar parameters are planned for a forthcoming paper.

Place, publisher, year, edition, pages
EDP Sciences, 2015
Keyword
Line: formation, atomic data, stellar abundances
National Category
Astronomy, Astrophysics and Cosmology Atom and Molecular Physics and Optics
Research subject
Physics with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-249165 (URN)10.1051/0004-6361/201525846 (DOI)000358877100065 ()
Available from: 2015-04-11 Created: 2015-04-11 Last updated: 2017-12-04Bibliographically approved
3. Mg line formation in late-type stellar atmospheres: II. Calculations in a grid of 1D models
Open this publication in new window or tab >>Mg line formation in late-type stellar atmospheres: II. Calculations in a grid of 1D models
(English)Manuscript (preprint) (Other academic)
Keyword
non-LTE, line: formation, stars: abundances
National Category
Astronomy, Astrophysics and Cosmology Atom and Molecular Physics and Optics
Research subject
Physics with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-249167 (URN)
Available from: 2015-04-11 Created: 2015-04-11 Last updated: 2015-07-07

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