Emission-line Helium Abundances in Highly Obscured Nebulae

It has long been possible to determine He/H ratios from radio or infrared recombination lines, making it possible to measure ionic abundance ratios for highly obscured emission-line objects. It is not possible to determine the total helium abundance from such data alone, however, since the ionization correction factor (ICF), the correction for unobservable stages of ionization of He or H, must also be determined. Optical forbidden lines are usually used for this, limiting studies to relatively unobscured objects. This paper outlines a way to determine the ICF using only infrared data. We identify four line pairs, [NeIII] 36 μm/[NeII] 12.8 μm, [NeIII] 15.6μm /[NeII] 12.8 μm, [ArIII] 9 μm/[ArII] 6.9 μm, and [ArIII] 21 μm/[ArII] 6.9 μm, 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 are formed within the He/H region of the nebula. We compute a very wide range of photoionization models appropriate for galactic HII regions. The models cover a wide range of densities, ionization parameters, 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 μm/[ArII] 6.9 μm ratio can indicate the ICF directly due to the near exact match in the critical densities of the two lines. Finally, Also at: Department of Astronomy, University of Toronto, Toronto, ON, Canada M5S 3H8 Also at: Department of Physics, University of Kentucky, Lexington, KY 405506-0055 USA