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

TL;DR

This study models the abundance of interstellar argonium (ArH$^+$) in diffuse clouds, revealing it mainly traces small, nearly atomic clouds with very low molecular fractions, distinct from other ions like OH$^+$ and H$_2$O$^+$.

Contribution

It provides a detailed parameter study predicting ArH$^+$ abundance and identifies it as a unique tracer of almost purely atomic interstellar gas, based on cloud size and molecular fraction.

Findings

ArH$^+$ primarily resides in small, low-extinction clouds.

ArH$^+$ traces nearly atomic gas with molecular fractions between 10$^{-5}$ and 10$^{-2}$.

Different ions occupy clouds with varying sizes and molecular content.

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 $\rm H_2O^+$ 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 $\rm H_2O^+$ 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.