Ubiquitous argonium (ArH+) in the diffuse ISM (P. Schilke et al.)
In section 6. Interstellar and circumstellar matter
Ubiquitous argonium (ArH+) in the diffuse interstellar medium: A molecular tracer of almost pure atomic gas
It is not entirely unexpected to find that a molecular ion is a tracer for diffuse atomic ISM clouds. It has been known for some time that ions such as HCO+ observed in absorption against the background of point-like radio sources trace low-density, mainly atomic material. More recently, ions such as H2O+ have been detected with Herschel. Nevertheless, it is a surprise to find that argonium (ArH+) can be detected not only in the Crab nebula but also (as shown in the article by Schilke et al.) in clouds along the line of sight to various strong submm sources in the Galactic plane. It is also surprising to find that, based on the diffuse cloud chemistry models, ArH+ likes to live in clouds with a molecular fraction 2n(H2)/n(HI) of approximately 10-3. All molecules are not alike!