A non-LTE study of neutral and singly-ionized calcium in late-type stars
By L. Mashonkina, A. Korn, N. Przybilla. 2006
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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 $\ion{Ca}{i}$ / $\ion{Ca}{ii}$ line-strength ratios as indicators of surface gravity for extremely metal-poor stars. Methods. A comprehensive model atom for $\ion{Ca}{i}$ and $\ion{Ca}{ii}$ is presented. Accurate radiative and electron collisional atomic data are incorporated. The role of inelastic collisions with hydrogen atoms in the statistical equilibrium of $\ion{Ca}{i/ii}$ is estimated empirically from inspection of their different influences on the $\ion{Ca}{i}$ and $\ion{Ca}{ii}$ 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 $\ion{Ca}{i}$ 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 $\ion{Ca}{i}$ $\lambda\,4226$ at [Ca/H] = -4.9. In contrast, the NLTE effects strengthen the $\ion{Ca}{ii}$ lines and lead to negative abundance corrections. NLTE corrections are small, ≤ 0.02 dex, for the $\ion{Ca}{ii}$ 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 $\ion{Ca}{i/ii}$ 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 $\ion{Ca}{i}$ and $\ion{Ca}{ii}$ give consistent abundances for all objects (except Procyon) when collisions with hydrogen atoms are taken into account. The derived absolute solar Ca abundance (from $\ion{Ca}{i}$ and $\ion{Ca}{ii}$ lines) is $\log\varepsilon_{\rm Ca,\odot}$ = 6.38 ± 0.06. For Procyon, the mean Ca abundance from $\ion{Ca}{i}$ 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 ($\ion{Ca}{i}$4226) / W ($\ion{Ca}{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 $\ion{Ca}{ii}$ resonance line(s).
Subfields: Astronomy and Astrophysics
View Paper: https://www.semanticscholar.org/paper/da164558daad7079c052534505d48dadebff9b01
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Citations in the corpus
- A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars ? (2011, background): “With a similar value of SH = 0.1, the ionization equilibrium between Ca i and Ca ii in selected metal-poor stars was matched consistently with surface gravities derived from Hipparcos parallaxes (Mashonkina et al. 2007).”
- High-precision abundances of elements in solar twin stars - Trends with stellar age and elemental condensation temperature (2015, unknown): “…a function ofTeff, logg, and [Fe/H] for the lines of C i (Takeda & Honda 2005), Mgi (Zhao & Gehren 2000), Si and Zni (Takeda et al. 2005), and Cai (Mashonkina et al. 2007), suggest similar small differential corrections as in the case of the Fei lines; hence the non-LTE eff ct on the…”
- The elemental composition of the Sun - I. The intermediate mass elements Na to Ca (2014, methodology): “For Ca ii, no very accurate g f -values exist for the permitted lines; we relied on the choices discussed by Mashonkina et al. (2007), with data ultimately coming from Opacity Project calculations under the assumption of LS coupling.”
- HIGH-RESOLUTION SPECTROSCOPY OF EXTREMELY METAL-POOR STARS FROM SDSS/SEGUE. I. ATMOSPHERIC PARAMETERS AND CHEMICAL COMPOSITIONS (2012, background): “Non-LTE effects on abundance measurements for extremely metal-poor stars were investi- gated for Mg by Andrievsky et al. (2010), and for Ca by Mashonkina et al. (2007) and Spite et al. (2012).”
- Gaia FGK benchmark stars: Effective temperatures and surface gravities (2015, background): “References: (1) Mashonkina et al. (2007), (2) Valenti & Fischer (2005), (3) Luck & Heiter (2005), (4) Luck & Heiter (2007).”
- Quantifying the uncertainties of chemical evolution studies II. Stellar yields (2010, background): “Departures from non-LTE in the line formation for both neutral and singly-ionized calcium have been recently investigated by Mashonkina et al. (2007), for a wide range of metallicities of the stars.”
- Atomic Diffusion and Mixing in Old Stars. I. Very Large Telescope FLAMES-UVES Observations of Stars in NGC 6397 (2007, methodology): “We therefore made an effort to model as many elements as possible using non-LTE line formation: Mg i is modelled following Gehren et al. (2004), Ca i following Mashonkina et al. (2007), Fe i following Korn et al. (2003) and Ba ii following Mashonkina et al. (1999).”