Kinetic equilibrium of iron in the atmospheres of cool dwarf stars I. The solar strong line spectrum

Line formation calculations of Fe I and Fell in the solar atmosphere are presented for atomic models of iron including all observed terms and line transitions with available f-values. Recent improved calculations of Fe I photoionization cross-sections are taken into account, and the influence of collision processes is investigated by comparing synthesized and observed solar line flux profiles. The background is represented by the opacity of ail important non-iron elements with iron lines added. Using a representative sample of sufficiently unblended strong Fe I and Fe II line profiles, it is evident that line formation is affected by (a) velocity fields and (b) deviations from local thermodynamic equilibrium (NLTE). The calculations are extended to a systematic analysis demonstrating that the ionization equilibrium of iron is recovered for solar parameters (T(eff) = 5780 K: log g = 4.44) either using the empirical atmospheric model of Holweger & Muller (1974) and assuming LTE for both FeI and Fe II or a line-blanketed theoretical atmospheric model with NLTE iron line formation. In the latter case the kinetic equilibrium of Fe I shows a substantial underpopulation of Fe I terms which depends sensitively on both the improved photoionization calculations and the choice of hydrogen collision rates while the Fe II ion is well approximated by LTE. Although the source functions of most of the Fe I lines are nearly thermal, their formation is shifted deeper into the photosphere. NLTE wings of strong Fe I lines are therefore shallower than under the LTE assumption, whereas the cores of the strongest lines display the usual chromospheric contributions. Based on both calculated and laboratory f-values the abundances of 37 Fe II lines range between log epsilon (Fe II,.) = 7.50 and 7.56, depending on atomic and atmospheric models, and those of 117 FeI lines between log epsilon (Fe II,.) = 7.47 and 7.56, both with a relatively large scatter of 0.08 ... 0.12. The collisional coupling of FeI levels is investigated. Electron collisions seem to play only a minor role. Hydrogen collisions are very important between terms of low excitation, and they efficiently thermalize the line source functions but not necessarily the populations of the lower levels that determine the line optical depth. Thermalization of those low-excitation terms that are responsible for most of the lines analyaed is achieved only if the collisional coupling among highly excited Fe I terms and their Fe II parent terms is increased by large factors compared with standard collision rates. Solar flux profiles are reproduced under the assumption of both LTE or NLTE, with nearly all types of atomic and atmospheric models, because the Fe ionization equilibrium depends on the corresponding sets of f-values.