Stellar iron abundances:: Non-LTE effects

We report new statistical equilibrium calculations for Fe I and Fe II in the atmosphere of late-type stars. We used atomic models for Fe I and Fe II having, respectively, 256 and 190 levels, as well as 2117 and 3443 radiative transitions. Photoionization cross sections are from the Iron Project. These atomic models were used to investigate non-LTE (NLTE) effects in iron abundances of late-type stars with different atmospheric parameters. We found that most Fe I lines in metal-poor stars are formed in conditions far from LTE. We derived metallicity corrections of about 0.3 dex with respect to LTE values for the case of stars with [Fe/H] similar to -3.0. Fe II is found not to be affected by significant NLTE effects. The main NLTE effect invoked in the case of Fe I is overionization by ultraviolet radiation; thus classical ionization equilibrium is far from being satisfied. An important consequence is that surface gravities derived by LTE analysis are in error and should be corrected before final abundance corrections. This apparently solves the observed discrepancy between spectroscopic surface gravities derived by LTE analyses and those derived from Hipparcos parallaxes. A table of NLTE [Fe/H] and log g values for a sample of metal-poor late-type stars is given.