The abundance of + in diffuse clouds

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.