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

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 $H_2{O}^{+}$ 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 ${10}^{-5}\text{--}{10}^{-2}$, while OH⁺ and $H_2{O}^{+}$ reside primarily in somewhat larger clouds with a column-averaged molecular fraction ∼0.2. This analysis confirms our previous suggestion that the argonium molecular ion is a unique tracer of almost purely atomic gas.