2007A&A...461..261M

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 CaI/CaII line-strength ratios as indicators of surface gravity for extremely metal-poor stars. A comprehensive model atom for CaI and CaII is presented. Accurate radiative and electron collisional atomic data are incorporated. The role of inelastic collisions with hydrogen atoms in the statistical equilibrium of CaI/CaII is estimated empirically from inspection of their different influences on the CaI and CaII lines in selected stars with well determined stellar parameters and high-quality observed spectra. The dependence of NLTE effects on the atmospheric parameters is discussed. Departures from LTE significantly affect the profiles of CaI lines over the whole range of stellar parameters being considered. However, at [Ca/H]≥-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 +0.5dex for CaI λ4226 at [Ca/H]=-4.9. In contrast, the NLTE effects strengthen the CaII lines and lead to negative abundance corrections. NLTE corrections are small, ≤0.02dex, for the CaII resonance lines, and they grow in absolute value with decreasing Ca abundance for the IR lines of multiplet 3d-4p, exceeding 0.4dex in the metal-poor models with [Fe/H]≤-3. As a test and first application of the CaI/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 CaI and CaII give consistent abundances for all objects (except Procyon) when collisions with hydrogen atoms are taken into account. The derived absolute solar Ca abundance (from CaI and CaII lines) is logε_Ca,☉=6.38±0.06. For Procyon, the mean Ca abundance from CaI lines is markedly subsolar, [Ca/H]=-0.14±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(CaI4226)/W(CaII8498) 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 CaII resonance line(s).