Theoretical models for H II regions. II - The extragalactic H II region abundance sequence

With the aid of an extensive homogeneous grid of theoretical photoionization models (Evans and Dopita, 1985), a multiline fit to the observed emission-line spectral sequence of extragalactic H II regions is generated. It is possible to produce an excellent fit between the (rather narrow) emission-line ratio sequences that are observed and the theoretical sequences, provided that the average ionization parameter is strongly coupled with metallicity, and provided that nitrogen is a product of secondary nucleosynthesis. Using this theoretical sequence of H II region models, the semiempirical abundance diagnostic ratios used by Alloin et al. (1979), Pagel et al. (1979), and McCall et al. (1985) are recalculated, and new abundance of diagnostic ratios are generated. It is found that the correlation between ionization parameter and abundance has the effect of changing the previous abundance calibration toward lower abundances, particularly for the most metal-rich H II regions. Finally, a new technique to derive abundances from H II region spectra is presented.