2000A&A...355..333L

We have taken new, broader-band and higher-resolution profiles of Galactic 1667 MHz OH and 89.2 GHz HCO⁺ absorption toward several compact, extragalactic mm-wave continuum sources. The profiles are generally stable - quite similar between epochs and between the two species - but with occasional time-variations and differences. Typical linewidths are 1.0km/s (FWHM) in either OH or HCO⁺ and there are no differences in mean velocity. Profiles are compound but do not show broad wings, multiplicity, assymmetry, or other phenomena strikingly indicative of formation under extraordinary circumstances, consistent with the low ambient thermal pressures reflected in the weak rotational excitation of CO and HCO⁺. However, we have also discovered the existence of a low-lying, broad component of HCO⁺ absorption covering just those portions of the spectrum where τ_HI≥0.1-0.2 at λ21cm. Toward B 0355+508 at b=-1.6°, HCO⁺ absorption extends continuously over more than 40km/s. The broadly-distributed HCO⁺ absorption can be understood in terms of the known molecular fraction of local gas, as long as HCO⁺ is generally present at about its typical abundance n(HCO⁺)/n(H₂)=2x10^–9. The fact that CO forms rapidly from HCO⁺ in diffuse gas then suffices to account for the abundance of CO in diffuse/translucent material over the entire range 10¹²≤N(CO)≤10¹⁶cm^–2, 10¹⁹≤H₂≤10²¹cm^–2, using otherwise standard cloud models. Using models of molecular formation and excitation and the H-H₂, C⁺-CO transition in diffuse gas, and noting the absence of HCO⁺ emission at levels of 0.02-0.05K, we show very directly that the line profile variations are not the result of AU-sized inclusions of high hydrogen volume density, in the manner usually inferred. Instead, it is necessary to account for small-scale chemical and other inhomogeneities.