Kinetic equilibrium of iron in the atmospheres of cool dwarf stars II. Weak Fe I lines in the solar spectrum

Abstract

NLTE line formation calculations of FeI in the solar atmosphere are extended to include weak optical lines. Previously established atomic models are used to discriminate between different ways of treating collisional interaction processes. To derive a common solar FeI abundance from both strong and weak lines, fine-tuning of the microturbulence velocity parameter and the van-der- Waals damping constants is required. The solar FeI abundances based on all available f-values are dominated by the large scatter already found for the stronger lines. In particular the bulk of the data from the work of May et al. and O'Brian et al. is not adequate for accurate abundance work. Based on f-values measured by the Hannover and Oxford groups alone, the FeI LTE abundances are eps(FeI,Sun)=7.57 for the empirical and eps(FeI,Sun) = 7.48 ... 7.51 for the line-blanketed solar model. The solar Fe ionization equilibrium obtained for different atomic and atmospheric models rules out NLTE atomic models with a low efficiency of hydrogen collisions. At variance with Paper I, it is now in better agreement with laboratory FeII f-values for all types of line-blanketed models. Our final model assumptions consistent with a single unique solar Fe abundance eps(Fe,Sun) = 7.48 ... 7.51 calculated from NLTE line formation are (a) a line-blanketed solar model atmosphere, (b) an iron model atom with hydrogen collision rates 0.5 < S_H < 5 times the standard value to compensate for the large photoionization cross-sections, (c) a microturbulence velocity xi = 1.0 kms, (d) van-der-Waals damping parameters decreased by Delta(log C6) = -0.10...-0.15 as compared to Anstee & O'Mara's calculations, depending on S_H, (e) FeII f-values as published by Schnabel et al., and (f) FeI f-values published by the Hannover and Oxford groups.