The abundance of HOC+ in diffuse clouds

Abstract

We used the Plateau de Bure Interferometer to search for $\lambda$3mm 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 those 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(\HH) $ = 3-6\times 10^{-11} $, can be made in quiescent diffuse gas at thermal gas-kinetic rates if the {\HH O}/OH ratio is of order unity, in mild violation of extant observational limits. %