Emission-line helium abundances in highly obscured nebulae

This paper outlines a way to determine the ionization correction factor (ICF) using only infrared data. We identify four line pairs, [Ne III] 36 mu m/[Ne II] 12.8 mu m, [Ne III] 15.6 mu m/[Ne II] 12.8 mu m, [Ar III] 9 mu m/[Ar II] 6.9 mu m, and [Ar III] 21 mu m/[Ar II] 6.9 mu m, that are sensitive to the He ICF. This happens because the ions cover a wide range of ionization, the line pairs are not sensitive to electron temperature, they have similar critical densities, and they are formed within the He+/H+ region of the nebula. We compute a very wide range of photoionization models appropriate for galactic H II regions. The models cover a wide range of densities, ionization parameters, and stellar temperatures and use continua from four very different stellar atmospheres. The results show that each line pair has a critical intensity ratio above which the He ICF is always small. Below these values the ICF depends very strongly on details of the models for three of the ratios, and so other information would be needed to determine the helium abundance. The [Ar III] 9 mu m/[Ar II] 6.9 mu m ratio can indicate the ICF directly as a result of the near exact match in the critical densities of the two lines. Finally, continua predicted by the latest generation of stellar atmospheres are sufficiently hard that they routinely produce significantly negative ICFs.