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

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. Results. Thanks to the detection of two isotopologues, (ArH+)-Ar-36 and (ArH+)-Ar-38, we are confident about the carrier assignment to 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 H-2 abundance of 10(-4)-10(-3) and shows little correlation to H2O+, which traces gas with a fractional H2 abundance of Conclusions. 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 H-2 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.