Oxygen Abundances in Metal‐poor Stars

We present oxygen abundances derived from both the permitted and forbiddenoxygen lines for 55 subgiants and giants with [Fe/H] values between -2.7 andsolar with the goal of understanding the discrepancy in the derived abundances.A first attempt, using \teff{} values from photometric calibrations and surfacegravities from luminosities, obtained agreement between the indicators forturn-off stars, but the disagreement was large for evolved stars. We find thatthe difference in the oxygen abundances derived from the permitted andforbidden lines is most strongly affected by \teff{}, and we derive a new\teff{} scale based on forcing the two sets of lines to give the same oxygenabundances. These new parameters, however, do not agree with other observables,such as theoretical isochrones or Balmer-line profile based \teff{}determinations. Our analysis finds that one-dimensional, LTE analyses (withpublished NLTE corrections for the permitted lines) cannot fully resolve thedisagreement in the two indicators without adopting a temperature scaleincompatible with other temperature indicators. We also find no evidence ofcircumstellar emission in the forbidden lines, removing such emission as apossible cause for the discrepancy.Comment: 41 pages, 12 tables, 17 figures. Accepted for publication in Oct. 2003 Ap