³He in the Milky Way Interstellar Medium: Ionization Structure

The cosmic abundance of the ³He isotope has important implications for many fields of astrophysics. We are using the 8.665 GHz hyperfine transition of ³He⁺ to determine the ³He/H abundance in Milky Way H II regions and planetary nebulae. This is one in a series of papers in which we discuss issues involved in deriving accurate ³He/H abundance ratios from the available measurements. Here we describe the ionization correction we use to convert the ³He⁺/H⁺ abundance, y, to the ³He/H abundance, y₃. In principle the nebular ionization structure can significantly influence the y₃ derived for individual sources. We find that in general there is insufficient information available to make a detailed ionization correction. Here we make a simple correction and assess its validity. The correction is based on radio recombination line measurements of H⁺ and ⁴He⁺, together with simple core-halo source models. We use these models to establish criteria that allow us to identify sources that can be accurately corrected for ionization and those that cannot. We argue that this effect cannot be very large for most of the sources in our observational sample. For a wide range of models of nebular ionization structure we find that the ionization correction factor varies from 1 to 1.8. Although larger corrections are possible, there would have to be a conspiracy between the density and ionization structure for us to underestimate the ionization correction by a substantial amount.