Non-LTE line formation of Fe in late-type stars - II. 1D spectroscopic stellar parameters

We investigate departures from local thermodynamic equilibrium (LTE) in the line formation of neutral and singly ionized iron lines and their impact on spectroscopic stellar parameters. The calculations were performed for an extensive grid of 1D MARCS models of metal-rich and metal-poor late-type dwarfs and giants. We find that iron abundances derived from Fe I lines are increasingly underestimated in hotter, lower surface gravity and more metal-poor stars, in a simple and well-defined pattern, while LTE is usually a realistic approximation for Fe II lines. For the vast majority of dwarfs and giants, the perturbed ionization balance of Fe I and Fe II is the main relevant non-LTE effect to consider in the determination of spectroscopic parameters, while for extremely metal-poor stars and hot giant stars significant impact is seen also on the excitation balance and on the microturbulence determination from Fe I lines.