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

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 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).

SYSTEMATIC NON-LTE STUDY OF THE −2.6 ≤ [Fe/H] ≤ 0.2 F AND G DWARFS IN THE SOLAR NEIGHBORHOOD. II. ABUNDANCE PATTERNS FROM Li TO Eu*

“…As discussed in detail by Mashonkina et al (2007a) and Mashonkina (2013), lines of the photoionization-dominated minority species Ca I and Mg I can have negative Δ NLTE in the close-to-solar-metallicity models and positive correction in the low-metallicity models (Ca I λλ5349, 5588 and Mg I λ5528 in Figure 2). Here, we review briefly.…”

“…principles of non-LTE (NLTE) line formation and codes capable of such calculations have been around for a long time but have only been explored in a more systematic fashion for a wide range of stellar parameters over the past decade or so (e.g., Zhao et al 1998;Zhao & Gehren 2000;Mashonkina & Gehren 2001;Takeda et al 2002;Gehren et al 2004;Fabbian et al 2006;Mashonkina & Zhao 2006;Zhang et al 2006b;Mashonkina et al 2007aMashonkina et al , 2008Bergemann & Gehren 2008;Zhang et al 2008;Lind et al 2009;Shi et al 2009;Andrievsky et al 2010;Bergemann & Cescutti 2010;Spite et al 2011Spite et al , 2012Yan et al 2015). Before the observations are used to give constraints to the GCE models, we need to take into account the analysis errors of the stellar abundances, such as the departures from LTE and uncertainties in the atmospheric parameters.…”

TOPoS

“…We provide the abundances implied by our spectra both for log g = 4.0 and log g = 4.3. (Spite et al 2012a;Korn et al 2009;Mashonkina et al 2007) and it is recognised that the effects on the Ca ii H and K lines is very small, while it can be significant for the Ca ii IR triplet.…”

“…The deviations from LTE in the formation of Ca i and Ca ii lines in metal-poor stars has been extensively studied by Mashonkina et al (2007) and Spite et al (2012a). It has been recognised that the Ca i 422.6 nm resonance line provides discrepant results even when computed in NLTE (Spite et al 2012a), for this reason we never used the abundance from this line even when it was measured.…”

Are ancient dwarf satellites the building blocks of the Galactic halo?

“…In particular, the data we use for Galactic halo stars are from Gratton et al (2003), Cayrel et al (2004), Akerman et al (2004), Mashonkina, Korn & Przybilla (2007) and Shi et al (2009). For Ba, we use the data of Frebel (2010), as selected and binned by Cescutti et al (2013).…”