H II region abundances - Model oxygen line ratios

We present an extensive, homogeneous grid of H II region models. We find 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 (O II λ3727 + O III 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. It results naturally from the stellar mass-temperature-metallicity relation. Also, we find no convincing evidence for a correlation between the ionization parameter and metallicity which cannot be attributed to observational selection effects. Hence it remains possible that the extragalactic H II region excitation sequence is purely a metallicity effect.