We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H] ≈ −2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellarparameter differences between groups of stars, are in good agreement with broad-band and Strömgren photometry calibrated on the infrared-flux method. The spectroscopic abundance analysis reveals, for the first time, systematic trends of iron abundance with evolutionary stage. Iron is found to be 31 % less abundant in the turnoff-point stars than in the red giants. An abundance difference in lithium is seen between the turnoff-point and warm subgiant stars. The impact of potential systematic errors on these abundance trends (stellar parameters, the hydrostatic and LTE approximations) is quantitatively evaluated and found not to alter our conclusions significantly. Trends for various elements (Li, Mg, Ca, Ti and Fe) are compared with stellar-structure models including the effects of atomic diffusion and radiative acceleration. Such models are found to describe the observed element-specific trends well, if extra (turbulent) mixing just below the convection zone is introduced. It is concluded that atomic diffusion and turbulent mixing are largely responsible for the sub-primordial stellar lithium abundances of warm halo stars. Other consequences of atomic diffusion in old metal-poor stars are also discussed.

For the first time, we present an extensive study of stars with individual non-LTE (NLTE) abundances for 17 chemical elements from Li to Eu in a sample of stars uniformly distributed over the −2.62 [Fe/H] +0.24 metallicity range that is suitable for the Galactic chemical evolution research. The star sample has been kinematically selected to trace the Galactic thin and thick disks and halo. We find new results and improve earlier ones as follows: (i) the element-to-iron ratios for Mg, Si, Ca, and Ti form a metal-poor (MP) plateau at a similar height of 0.3 dex, and the knee occurs at common

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 Ca i / Ca ii line-strength ratios as indicators of surface gravity for extremely metal-poor stars.Methods. A comprehensive model atom for Ca i and Ca 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 Ca i/ii is estimated empirically from inspection of their different influences on the Ca i and Ca 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 Ca i 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 +0.5 dex for Ca i λ 4226 at [Ca/H] = −4.9. In contrast, the NLTE effects strengthen the Ca ii lines and lead to negative abundance corrections. NLTE corrections are small, ≤0.02 dex, for the Ca 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] ≤ −3. As a test and first application of the Ca 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 Ca i and Ca 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 Ca i and Ca ii lines) is log ε Ca, = 6.38 ± 0.06. For Procyon, the mean Ca abundance from Ca i 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(Ca i4226) / W(Ca ii8498) 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 Ca ii resonance line(s).