Astronomy & Astrophysics

Issue		A&A Volume 461, Number 1, January I 2007
Page(s)		261 - 275
Section		Stellar atmospheres
DOI		10.1051/0004-6361:20065999

A&A 461, 261-275 (2007)
DOI: 10.1051/0004-6361:20065999

A non-LTE study of neutral and singly-ionized calcium in late-type stars

L. Mashonkina^{1, 2}, A. J. Korn³, and N. Przybilla⁴

¹ Institut für Astronomie und Astrophysik der Universität München, Scheinerstr. 1, 81679 München, Germany
e-mail: lyuda@usm.lmu.de
² Institute of Astronomy, Russian Academy of Science, Pyatnitskaya 48, 119017 Moscow, Russia
e-mail: lima@inasan.ru
³ Department of Astronomy and Space Physics, Uppsala University, Box 515, 75120 Uppsala, Sweden
⁴ Dr. Remeis-Sternwarte Bamberg, Sternwartstrasse 7, 96049 Bamberg, Germany

(Received 10 July 2006 / Accepted 4 September 2006 )

Abstract
Aims.Non-local thermodynamical equilibrium (NLTE) line formation for neutral and singly-ionized calcium is considered through a range of spectral types when the Ca abundance varies from the solar value down to [Ca/H] = -5. We evaluate the influence of departures from LTE on Ca abundance determinations and inspect the possibility of using $\ion{i}$ / $\ion{ii}$ line-strength ratios as indicators of surface gravity for extremely metal-poor stars.
Methods.A comprehensive model atom for $\ion{i}$ and $\ion{ii}$ is presented. Accurate radiative and electron collisional atomic data are incorporated. The role of inelastic collisions with hydrogen atoms in the statistical equilibrium of $\ion{i/ii}$ is estimated empirically from inspection of their different influences on the $\ion{i}$ and $\ion{ii}$ lines in selected stars with well determined stellar parameters and high-quality observed spectra.
Results.The dependence of NLTE effects on the atmospheric parameters is discussed. Departures from LTE significantly affect the profiles of $\ion{i}$ lines over the whole range of stellar parameters being considered. However, at [Ca/H] $\ge$ -2, NLTE abundance correction of individual lines have a low absolute value due to the different influence of NLTE effects on line wings and the line core. At lower Ca abundances, NLTE leads to systematically depleted total absorption in the line and positive abundance corrections, exceeding +0.5 dex for $\ion{i}$ $\lambda\,4226$ at [Ca/H] = -4.9. In contrast, the NLTE effects strengthen the $\ion{ii}$ lines and lead to negative abundance corrections. NLTE corrections are small, $\le$ 0.02 dex, for the $\ion{ii}$ resonance lines, and they grow in absolute value with decreasing Ca abundance for the IR lines of multiplet 3d-4p, exceeding 0.4 dex in the metal-poor models with [Fe/H] $\le$ -3. As a test and first application of the $\ion{i/ii}$ model atom, Ca abundances are determined on the basis of plane-parallel LTE model atmospheres for the Sun, Procyon (F IV-V), and seven metal-poor stars, using high S/N and high-resolution spectra at visual and near-IR wavelengths. Lines of $\ion{i}$ and $\ion{ii}$ give consistent abundances for all objects (except Procyon) when collisions with hydrogen atoms are taken into account. The derived absolute solar Ca abundance (from $\ion{i}$ and $\ion{ii}$ lines) is $\log\varepsilon_{\rm Ca,\odot}$ = 6.38 $\pm$ 0.06. For Procyon, the mean Ca abundance from $\ion{i}$ lines is markedly subsolar, [Ca/H] = -0.14 $\pm$ 0.03. All metal-poor stars within our sample show an overabundance of calcium relative to iron with [Ca/Fe] abundance ratios of 0.26 to 0.46 that are typical of the halo population. The W( $\ion{i}$ 4226) / W( $\ion{ii}$ 8498) equivalent width ratio is predicted to be sensitive to surface gravity for extremely metal-poor stars, while this is not the case for the ratio involving the $\ion{ii}$ resonance line(s).

Key words: line: formation -- Sun: atmosphere -- stars: abundances -- stars: late-type -- line: profiles

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