The ionization equilibrium of iron in H II regions

We study the ionization equilibrium of Fe using photoionization models that incorporate improved values for the ionization and recombination cross-sections and the charge-exchange rates for the Fe ions. The previously available photoionization models predict concentrations of Fe3+ which are a factor of 3-8 higher than the values inferred from emission lines of [Fe III] and [Fe IV]. Our new models reduce these discrepancies to factors of 2-5. We discuss the possible reasons behind the remaining discrepancies and present an updated ionization correction factor for obtaining the Fe abundance from the Fe++ abundance.