The chemistry of interstellar argonium and other probes of the molecular fraction in diffuse clouds

Abstract

We present a general parameter study, in which the abundance of interstellar argonium (ArH${}^{+}$) is predicted using a model for the physics and chemistry of diffuse interstellar gas clouds. Results have been obtained as a function of UV radiation field, cosmic-ray ionization rate, and cloud extinction. No single set of cloud parameters provides an acceptable fit to the typical ArH${}^{+}$, OH${}^{+}$ and ${\mathrm{H}}_2{\mathrm{O}}^{+}$ abundances observed in diffuse clouds within the Galactic disk. Instead, the observed abundances suggest that ArH${}^{+}$ resides primarily in a separate population of small clouds of total visual extinction of at most 0.02 mag per cloud, within which the column-averaged molecular fraction is in the range $10^{-5}-10^{-2}$, while OH${}^{+}$ and ${\mathrm{H}}_2{\mathrm{O}}^{+}$ reside primarily in somewhat larger clouds with a column-averaged molecular fraction $\sim 0.2$. This analysis confirms our previous suggestion that the argonium molecular ion is a unique tracer of almost purely atomic gas.

Accepted for publication in ApJ. 26 pages, 8 figures

References in corpus (6)

• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• Ubiquitous argonium (ArH${}^{+}$) in the diffuse interstellar medium -- a molecular tracer of almost purely atomic gas
• Sulphur-bearing molecules in diffuse molecular clouds: new results from SOFIA/GREAT and the IRAM 30 m telescope
• [CII] absorption and emission in the diffuse interstellar medium across the Galactic Plane
• A New Determination of the Binding Energy of Atomic Oxygen on Dust Grain Surfaces: Experimental Results and Simulations
• HD/H2 as a probe of the roles of gas, dust, light, metallicity and cosmic rays in promoting the growth of molecular hydrogen in the diffuse interstellar medium

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