The abundance of ⁺ in diffuse clouds

¹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA e-mail: hliszt@nrao.edu
² Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin-d'Hères, France
³ Chalmers Centre for Astrophysics and Space Science, Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden

Received: 12 July 2004
Accepted: 23 August 2004

Abstract

We used the Plateau de Bure Interferometer to search for λ3mm absorption lines of HOC⁺ from local diffuse and translucent clouds occulting compact extragalactic mm-wave continuum sources. We detected HOC⁺ in three directions with column densities only 70-120 times below that of the HCO⁺ isomer, a factor 5-50 higher than typically found in dense dark gas but comparable to recent observations of dense photon-dominated regions. The observed amounts of HOC⁺  N(HOC⁺)/N(H₂)  can be made in quiescent diffuse gas at thermal gas-kinetic rates if the H₂O/OH ratio is of order unity, in mild violation of extant observational limits.