NON-LTE EFFECT ON CNO ABUNDANCE DETERMINATIONS FOR SOLAR-TYPE STARS - THE SUN, PROCYON, AND THE OXYGEN PROBLEM IN METAL-POOR DWARFS

Statistical equilibrium calculations with realistic atomic models of C, N, and O were performed to clarify how the non-LTE effect plays a role in determining the photospheric CNO abundances (from lines of neutral atoms in the optical/IR region) for the Sun and Procyon. In addition, it was also examined to which amount the derived abundances are affected by a change in the collisional rates/microturbulence/damping parameter/model atmosphere, as well as by the molecule formation effect. The resulting non-LTE abundance corrections (\DELTA log epsilon\) turned out to be less than or similar 0.1 dex for most of the lines for both stars, which are regarded as being not very significant and being more or less comparable to the uncertainties involved in the several factors mentioned above. The CNO abundances of Procyon were confirmed to be practically the same as the solar compositions. Special attention was paid to the oxygen triplet lambda7771-5, which was found to show only an insignificant amount of the non-LTE effect by \DELTA log epsilon\ approximately 0.06 for the Sun (being less than those previously reported by other investigators), though an appreciably large correction of approximately 0.4 dex was derived for Procyon. A reasonable interpretation was found for the observed center to limb variation of the strength of this triplet in the Sun. Furthermore, it was revealed that the controversial problem regarding the well-known discrepancy between the oxygen abundances derived from the lambda7771-5 and lambda6300 lines in population II F-G dwarfs is satisfactorily explained by the non-LTE effect predicted by the present calculations.