2007A&A...462..535Y

Our aims are to estimate the validity of empirical methods, such as R₂₃, R₂₃-P, log([NII]/Hα) (N2), log[([OIII]/Hβ)/([NII]/Hα)] (O3N2), and log([SII]/Hα) (S2), and to re-derive (or add) the calibrations of R₂₃, N2, O3N2, and S2 indices for oxygen abundances on the basis of a large sample of galaxies with T_e-based abundances. We determined the gas-phase oxygen abundance for a sample of 695 galaxies and HII regions with reliable detections of [OIII]4363, using the reliable and direct temperature-sensitive (T_e) method of measuring metallicity. We selected 531 star-forming galaxies from the SDSS-DR4 database with strong emission lines, including [OIII]4363 detected at a signal-to-noise ratio higher than 5σ, as well as 164 galaxies and HII regions from the literature with T_e measurements. The O/H abundances were derived from a two-zone model for the temperature structure, assuming a relationship between high ionization and low ionization species. We compare our (O/H)_Te measurements of the SDSS sample with the abundances obtained by the MPA/JHU group who used multiple strong emission lines and Bayesian techniques (Tremonti et al., 2004ApJ...613..898T). For roughly half of the sample the Bayesian abundances are overestimated ∼0.34dex, possibly due to the treatment of nitrogen enrichment in the models they used. The R₂₃ and R₂₃-P methods systematically overestimate the O/H abundance by a factor of ∼0.20dex and ∼0.06dex, respectively. The N2 index, rather than the O3N2 index, provides more consistent O/H abundances with the T_e-method, but with some scatter. The relations of N2, O3N2, and S2 with log(O/H) are consistent with the photoionization model calculations of Kewley & Doptita (2002ApJS..142...35K), but R₂₃ does not match well. We derive analytical calibrations for O/H from R₂₃, N2, O3N2, and S2 indices on the basis of this large sample, including the excitation parameter P as an additional parameter in the N2 calibration. These empirical calibrations are free of the systematic problems inherent in abundance calibrations based on photoionization models. We conclude that the N2, O3N2, and S2 indices are useful indicators for calibrating metallicities of galaxies with 12+log(O/H)<8.5 and that the R₂₃ index works well for the metal-poor galaxies with 12+log(O/H)<7.9. For the intermediate metallicity range (7.9<12+log(O/H)<8.4), the R₂₃ and R₂₃-P methods are unreliable for characterizing the O/H abundances.