Using Strong Lines to Estimate Abundances in Extragalactic H ii Regions and Starburst Galaxies

We have used a combination of stellar population synthesis andphotoionization models to develop a set of ionization parameter and abundancediagnostics based only on the use of the strong optical emission lines. Thesemodels are applicable to both extragalactic HII regions and star-forminggalaxies. We show that, because our techniques solve explicitly for both theionization parameter and the chemical abundance, the diagnostics presented hereare an improvement on earlier techniques based on strong emission-line ratios.Our techniques are applicable at all metallicities. In particular, formetallicities above half solar, the ratio [NII]/[OII] provides a very reliablediagnostic since it is ionization parameter independant and does not have alocal maximum. This ratio has not been used historically because of worriesabout reddening corrections. However, we show that the use of classicalreddening curves is quite sufficient to allow this [NII]/[OII] diagnostic to beused with confidence as a reliable abundance indicator. The cause of systematicand random errors inherent in previous techniques are discussed, and we presenta new `optimal' abundance diagnostic method based on the use of line ratiosinvolving [NII], [OII], [OIII], [SII] and the Balmer lines. This combineddiagnostic appears to suffer no apparent systematic errors, can be used overthe entire abundance range and significantly reduces the random error inherentin previous techniques. Finally, we give a recommended procedure for thederivation of abundances in the case that only spectra of limited wavelengthcoverage are available so that the optimal method can no longer be used.Comment: 29 pages, 22 figures, 4 tables. Accepted for publication in ApJ