A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars (1101.4570v1)

Abstract: A comprehensive model atom for Fe with more than 3000 energy levels is presented. As a test and first application of this model atom, Fe abundances are determined for the Sun and five stars with well determined stellar parameters and high-quality observed spectra. Non-LTE leads to systematically depleted total absorption in the Fe I lines and to positive abundance corrections in agreement with the previous studies, however, the magnitude of non-LTE effect is smaller compared to the earlier results. Non-LTE corrections do not exceed 0.1 dex for the solar metallicity and mildly metal-deficient stars, and they vary within 0.21 dex and 0.35 dex in the very metal-poor stars HD 84937 and HD 122563, respectively, depending on the assumed efficiency of collisions with hydrogen atoms. Based on the analysis of the Fe I/Fe II ionization equilibrium in these two stars, we recommend to apply the Drawin formalism in non-LTE studies of Fe with a scaling factor of 0.1. For the Fe II lines, non-LTE corrections do not exceed 0.01 dex in absolute value. The solar non-LTE abundance obtained from 54 Fe I lines is 7.56+-0.09 and the abundance from 18 Fe II lines varies between 7.41+-0.11 and 7.56+-0.05 depending on the source of the gf-values. Thus, gf-values available for the iron lines are not accurate enough to pursue high-accuracy absolute abundance determinations. Lines of Fe I give, on average, a 0.1 dex lower abundance compared to those of Fe II lines for HD 61421 and HD 102870, even when applying a differential analysis relative to the Sun. A disparity between Fe I and Fe II points to problems of stellar atmosphere modelling or/and effective temperature determination.

Non-LTE Iron Line Spectroscopy in 1D Stellar Atmospheres

The paper addresses the non-local thermodynamic equilibrium (non-LTE) line formation of neutral and singly-ionized iron in the atmospheres of the Sun and late-type stars using one-dimensional (1D) models. Utilizing an extensive model atom with many measured and predicted levels, the authors evaluate iron abundances and ionization equilibrium, underscoring the importance of accurate model components and collision processes.

Incorporating data from both laboratory and theoretical predictions, the comprehensive model atom includes over 3,000 energy levels. This advancement achieves a close collisional coupling of high-excitation levels to the ground state of singly-ionized iron, enhancing the treatment of statistical equilibrium compared to earlier studies.

Key Findings

• Non-LTE Effects: The paper confirms that non-LTE conditions result in generally weaker \ion{Fe}{i} lines compared to their LTE strengths, especially for low-metallicity stars. For \ion{Fe}{ii}, deviations from LTE remain negligible under most conditions.
• Importance of Hydrogen Collisions: The paper empirically evaluates the role of inelastic collisions with hydrogen atoms in establishing statistical equilibrium. A scaling factor of 0.1 is recommended for calculations involving hydrogen collisions based on these interactions' effects on \ion{Fe}{i} and \ion{Fe}{ii} lines.
• Iron Abundance Determinations: Non-LTE corrections for iron abundance generally do not exceed 0.1 dex for solar and mildly metal-deficient stars. More substantial corrections, up to 0.35 dex, are observed in very metal-poor stars, depending on assumptions regarding hydrogen collisions.
• Solar Abundances: For the Sun, non-LTE conditions lead to an average \ion{Fe}{i} abundance of 7.56 with a minor correction of 0.03 dex. Abundance determinations based on \ion{Fe}{ii} are influenced by the choice of oscillator strengths, with a variance depending on sources of these atomic data.

Implications

This research has both practical and theoretical ramifications. Accurate modeling of iron lines under non-LTE conditions is crucial for retrieving reliable stellar parameters and chemical compositions, especially in metal-poor regimes. The suggested scaling for hydrogen collision effects, albeit empirical, guides future non-LTE studies in ambient stellar atmospheres, especially in cool and metal-poor stars. The reliability of oscillator strengths remains a challenge, impacting absolute element abundances. Thus, improved atomic data are vital for advancing stellar physics and our understanding of stellar and galactic evolution.

Future Directions

Further theoretical work is essential to characterize collisional processes comprehensively, not limited to hydrogenic collisions. Improved approaches toward 3D non-LTE modeling, although computationally demanding, promise more accurate description of stellar atmospheres, addressing discrepancies remaining in currently used 1D models.

This paper's methodology and findings provide a robust framework for future research aimed at refining stellar atmosphere models, crucial for probing the chemical evolution of galaxies and interpreting spectroscopic observations across a wide range of stellar environments.