Opened 3 years ago

#337 new enhancement

atomic model for Mg I

Reported by: Gary J. Ferland Owned by: nobody
Priority: minor Milestone:
Component: atomic/molecular data base Version: trunk
Keywords: Cc:


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


Mg line formation in late-type stellar atmospheres. I. The model atom
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.

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