Ubiquitous argonium (ArH⁺) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas

This is the authors' abstract. We don't add key points for this paper.

Aims. We describe the assignment of a previously unidentified interstellar absorption line to ArH + and discuss its relevance in the context of hydride absorption in diffuse gas with a low H 2 fraction. The confidence of the assignment to ArH + is discussed, and the column densities are determined toward several lines of sight. The results are then discussed in the framework of chemical models, with the aim of explaining the observed column densities. Methods. We fitted the spectral lines with multiple velocity components, and determined column densities from the line-to-continuum ratio. The column densities of ArH + were compared to those of other species, tracing interstellar medium (ISM) components with different H 2 abundances. We constructed chemical models that take UV radiation and cosmic ray ionization into account.