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.

arXiv commentAccepted for publication in ApJ. 26 pages, 8 figures

References in corpus (21)

• The UMIST database for astrochemistry 2012
• The Dark Molecular Gas
• Investigating the Cosmic-Ray Ionization Rate in the Galactic Diffuse Interstellar Medium through Observations of H3+
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• Interstellar Carbon in Translucent Sightlines
• Detection of a Noble Gas Molecular Ion, {36}ArH^+, in the Crab Nebula
• The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions
• Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction
• Ubiquitous argonium (ArH${}^{+}$) in the diffuse interstellar medium -- a molecular tracer of almost purely atomic gas
• The Homogeneity of Interstellar Oxygen in the Galactic Disk
• 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
• H2 distribution during formation of multiphase molecular clouds
• Detection of extragalactic argonium, ArH${}^{+}$, toward PKS~1830$-$211
• Herschel observations of interstellar chloronium
• A New Determination of the Binding Energy of Atomic Oxygen on Dust Grain Surfaces: Experimental Results and Simulations
• CII in the Interstellar Medium: Excitation by H2 Revisited
• Dissociative recombination measurements of HCl+ using an ion storage ring
• 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
• Extended Carbon Line Emission in the Galaxy: Searching for Dark Molecular Gas along the G328 Sightline
• Merged-beams Reaction Studies of O + H_3^+

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