Stellar Iron Abundances: Non‐LTE Effects

We report new statistical equilibrium calculations for Fe I and Fe II in theatmosphere of Late-Type stars. We used atomic models for Fe I and Fe II havingrespectively 256 and 190 levels, as well as 2117 and 3443 radiativetransitions. Photoionization cross-sections are from the Iron Project. Theseatomic models were used to investigate non-LTE effects in iron abundances ofLate-Type stars with different atmospheric parameters. We found that most Fe I lines in metal-poor stars are formed in conditionsfar from LTE. We derived metallicity corrections of about 0.3 dex with respectto LTE values, for the case of stars with [Fe/H] ~ -3.0. Fe II is found not tobe affected by significant non-LTE effects. The main non-LTE effect invoked inthe case of Fe I is overionization by ultraviolet radiation, thus classicalionization equilibrium is far to be satisfied. An important consequence is thatsurface gravities derived by LTE analysis are in error and should be correctedbefore final abundances corrections. This apparently solves the observed discrepancy between spectroscopic surfacegravities derived by LTE analyses and those derived from Hipparcos parallaxes.A table of non-LTE [Fe/H] and log g values for a sample of metal-poor late-typestars is given.Comment: 22 pages, 9 figures, 1 table, ApJ style, accepte