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 H2O+ 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 H2O+ reside primarily in somewhat larger clouds with a column-averaged molecular fraction similar to 0.2. This analysis confirms our previous suggestion that the argonium molecular ion is a unique tracer of almost purely atomic gas.