2014A&A...566A..29S

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 H₂ 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. 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 H₂ abundances. We constructed chemical models that take UV radiation and cosmic ray ionization into account. Thanks to the detection of two isotopologues, ³⁶ArH⁺ and ³⁸ArH⁺, we are confident about the carrier assignment to ArH⁺. NeH⁺ is not detected with a limit of [NeH⁺]/[ArH⁺]≤0.1. The derived column densities agree well with the predictions of chemical models. ArH⁺ is a unique tracer of gas with a fractional H₂ abundance of 10^–4-10^–3 and shows little correlation to H₂O⁺, which traces gas with a fractional H₂ abundance of ≃0.1. A careful analysis of variations in the ArH⁺, OH⁺, H₂O⁺, and HF column densities promises to be a faithful tracer of the distribution of the H₂ fractional abundance by providing unique information on a poorly known phase in the cycle of interstellar matter and on its transition from atomic diffuse gas to dense molecular gas traced by CO emission. Abundances of these species put strong observational constraints upon magnetohydrodynamical (MHD) simulations of the interstellar medium, and potentially could evolve into a tool characterizing the ISM. Paradoxically, the ArH⁺ molecule is a better tracer of almost purely atomic hydrogen gas than Hi itself, since Hi can also be present in gas with a significant molecular content, but ArH⁺ singles out gas that is >99.9% atomic.