A&A 566, A29 (2014) Astronomy DOI: 10.1051/0004-6361/201423727 & c ESO 2014 Astrophysics Ubiquitous argonium (ArH+) in the diffuse interstellar medium: A molecular tracer of almost purely atomic gas P. Schilke1, D. A. Neufeld2, H. S. P. Müller1, C. Comito1, E. A. Bergin3, D. C. Lis4;5, M. Gerin6, J. H. Black7, M. Wolfire8, N. Indriolo2, J. C. Pearson9, K. M. Menten10, B. Winkel10, Á. Sánchez-Monge1, T. Möller1, B. Godard6, and E. Falgarone6 1 I. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany e-mail:
[email protected] 2 The Johns Hopkins University, Baltimore, MD 21218, USA 3 Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA 4 California Institute of Technology, Pasadena, CA 91125, USA 5 Sorbonne Universités, Université Pierre et Marie Curie, Paris 6, CNRS, Observatoire de Paris, UMR 8112 LERMA, Paris, France 6 LERMA, CNRS UMR 8112, Observatoire de Paris & École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France 7 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden 8 Astronomy Department, University of Maryland, College Park, MD 20742, USA 9 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 10 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany Received 28 February 2014 / Accepted 29 March 2014 ABSTRACT 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 H2 fraction.