Title:
Mg line formation in late-type stellar atmospheres. I. The model atom
Authors:
Osorio, Y.; Barklem, P. S.; Lind, K.; Belyaev, A. K.; Spielfiedel, A.; Guitou, M.; Feautrier, N.
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 ?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 ?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.
{
"status": "new",
"changetime": "2019-02-04T13:07:06Z",
"_ts": "1549285626580696",
"description": "Gives collision data for Mg I, summary of data sources for Mg I and Mg II\n\nWe have H0 and e collisions from previous papers by this group in stout\n\nhttp://adsabs.harvard.edu/abs/2015A%26A...579A..53O\n\n2015A&A...579A..53O \n\n{{{\nTitle:\t\nMg line formation in late-type stellar atmospheres. I. The model atom\nAuthors:\t\nOsorio, Y.; Barklem, P. S.; Lind, K.; Belyaev, A. K.; Spielfiedel, A.; Guitou, M.; Feautrier, N.\n\n\nContext. Magnesium is an element of significant astrophysical importance, \noften traced in late-type stars using lines of neutral magnesium, which is \nexpected to be subject to departures from local thermodynamic equilibrium \n(LTE). The importance of Mg , together with the unique range of spectral \nfeatures in late-type stars probing different parts of the atom, as well as its \nrelative simplicity from an atomic physics point of view, makes it a prime target \nand test bed for detailed ab initio non-LTE modelling in stellar atmospheres. \nPrevious non-LTE modelling of spectral line formation has, however, been \nsubject to uncertainties due to lack of accurate data for inelastic collisions with \nelectrons and hydrogen atoms. \n\nAims: In this paper we build and test a Mg model atom for spectral line \nformation in late-type stars with new or recent inelastic collision data and no \nassociated free parameters. We aim to reduce these uncertainties and thereby \nimprove the accuracy of Mg non-LTE modelling in late-type stars. \n\nMethods: For the low-lying states of Mg i, electron collision data were \ncalculated using the R-matrix method. Hydrogen collision data, including \ncharge transfer processes, were taken from recent calculations by some of us. \nCalculations for collisional broadening by neutral hydrogen were also performed \nwhere data were missing. These calculations, together with data from the \nliterature, were used to build a model atom. This model was then employed in \nthe context of standard non-LTE modelling in 1D (including average 3D) model \natmospheres in a small set of stellar atmosphere models. First, the modelling \nwas tested by comparisons with observed spectra of benchmark stars with \nwell-known parameters. Second, the spectral line behaviour and uncertainties \nwere explored by extensive experiments in which sets of collisional data were \nchanged or removed. \n\nResults: The modelled spectra agree well with observed spectra from \nbenchmark stars, showing much better agreement with line profile shapes than \nwith LTE modelling. The line-to-line scatter in the derived abundances shows \nsome improvements compared to LTE (where the cores of strong lines must \noften be ignored), particularly when coupled with averaged 3D models. The \nobserved Mg emission features at 7 and 12 ?m in the spectra of the Sun and \nArcturus, which are sensitive to the collision data, are reasonably well \nreproduced. Charge transfer with H is generally important as a thermalising \nmechanism in dwarfs, but less so in giants. Excitation due to collisions with H is \nfound to be quite important in both giants and dwarfs. The R-matrix \ncalculations for electron collisions also lead to significant differences compared \nto when approximate formulas are employed. The modelling predicts non-LTE \nabundance corrections ?A(Mg )NLTE-LTE in dwarfs, both solar metallicity and \nmetal-poor, to be very small (of order 0.01 dex), even smaller than found in \nprevious studies. In giants, corrections vary greatly between lines, but can be \nas large as 0.4 dex. \n\nConclusions: Our results emphasise the need for accurate data of Mg collisions \nwith both electrons and H atoms for precise non-LTE predictions of stellar \nspectra, but demonstrate that such data can be calculated and that ab initio \nnon-LTE modelling without resort to free parameters is possible. In contrast to \nLi and Na, where only the introduction of charge transfer processes has led to \ndifferences with respect to earlier non-LTE modelling, the more complex case of \nMg finds changes due to improvements in the data for collisional excitation by \nelectrons and hydrogen atoms, as well as due to the charge transfer processes. \nGrids of departure coefficients and abundance corrections for a range of stellar \nparameters are planned for a forthcoming paper.\n\n}}}\n",
"reporter": "gary",
"cc": "",
"resolution": "",
"time": "2015-07-12T21:59:25Z",
"component": "atomic/molecular data base",
"summary": "atomic model for Mg I",
"priority": "minor",
"keywords": "",
"version": "trunk",
"milestone": "no milestone",
"owner": "nobody",
"type": "enhancement"
}
reported by: @CloudyLex
Gives collision data for Mg I, summary of data sources for Mg I and Mg II
We have H0 and e collisions from previous papers by this group in stout
http://adsabs.harvard.edu/abs/2015A%26A...579A..53O
2015A&A...579A..53O
Migrated from https://www.nublado.org/ticket/337