3-He in the Milky Way Interstellar Medium: Ionization Structure

Abstract

The cosmic abundance of the 3-He isotope has important implications for many fields of astrophysics. We are using the 8.665 GHz hyperfine transition of 3-He+ to determine the 3-He/H abundance in Milky Way HII regions and planetary nebulae. This is one in a series of papers in which we discuss issues involved in deriving accurate 3-He/H abundance ratios from the available measurements. Here we describe the ionization correction we use to convert the 3-He+/H+ abundance, y3+, to the 3-He/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 4-He+, 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 large 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.

36 pages, 4 figures To appear Astrophysical Journal, 20 August 2007, vol 665, no 2

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Cited by in corpus (6)

• Green Bank Telescope Observations of 3He+: HII Regions
• First Detection of ${}^3$He${}^{+}$ in the Planetary Nebula IC418
• Do All Low-Mass Stars Undergo Extra Mixing Processes?
• 3He: Does the problem persists?
• On the Observability of Optically Thin Coronal Hyperfine Structure Lines
• ${}^3$He Abundances in Planetary Nebulae