Oxygen abundance determination in very metal-poor giants: permitted OI lines versus forbidden [O I] lines

The abundance of oxygen was determined for selected very metal-poor G-K sta rs (six giants and one turn-off star) based on the high S/N and high-resolu tion spectra observed with the Keck HIRES in the red through near-IR region comprising the permitted O (I) lines (7771-5, 8446) along with the [O (I)] forbidden line at 6363 A. It turned out that both the abundances from the permitted line features, O (I) 17771-5 and O (I) 18446, agree quite well wi th each other, while the forbidden line yields somewhat discrepant and dive rgent abundances with a tendency of being underestimated on the average. Th e former (7773/8446) solution, which we believe to be more reliable, gives a fairly tight [O/Fe] vs. [Fe/H] relation such that increasing steadily fro m [O/Fe]similar to0.6 (at [Fe/H]similar to -1.5) to [O/Fe]similar to1.0 (at [Fe/H]similar to -3.0) is reasonably consistent with the trend recently re ported based on the analysis of the UV OH lines. (C) 2001 Elsevier Science B.V. All rights reserved.