H II region abundances - Model oxygen line ratios

An extensive, homogeneous grid of H II region models, is presented. It is found that the behavior of the strong oxygen lines can be modeled by taking proper account of the softening of the ionizing spectra produced by stars of increasing metallicity. This provides a calibration of the abundance indicating line ratio ([OII] λ3727 + [OIII] λλ4959, 5007)/Hβ which is comparable in accuracy to that obtainable by direct methods when a temperature-sensitive line ratio is available. No metallicity dependence of the IMF is required to explain the systematic softening of stellar spectra with increasing metallicity.