2001A&A...366..662N

Diffuse clouds embedded in a flowing intercloud medium will develop warm interface layers in which the thermal pressures should significantly exceed those within the cloud. We have investigated the gas-phase formation of polyatomic molecules within such warm interfaces. If an interface occupies a few percent of the total molecular column density along that line-of-sight, then many polyatomic species in that interface should have detectable abundances. We have compared the results of interface models with observational data on polyatomic species obtained by Liszt and Lucas. Some models give results that are in harmony with measured ratios of several species including C₂H, HCO⁺ and OH; these models have background radiation fields intensities that are lower than the standard by an order of magnitude. The number densities of molecular hydrogen are of the order of 10²cm^–3 in regions with temperatures of several thousand degrees. Such conditions in an interface do not conflict with the lower pressures inferred from CO data, as most of the CO is expected to be in the cold bulk of the material rather than in interfaces. The constancy of the sum of MHD wave ``pressure" and thermal pressure implies higher thermal pressures in interfaces than in the cold cloud and cold clump material. The radiation and thermal pressure conditions required to explain the observed abundance ratios may be more likely in interfaces around diffuse clumps within Giant Molecular Clouds than those around the isolated ``field" diffuse clouds observed in ultraviolet absorption. Some measured abundance ratios, the most notable of which is that of the abundances of H₂CO and HCO⁺, are not well matched by model results, though on the basis of the models the relevant species would be expected to be observable in some cases. It is possible that the adopted chemical network is incomplete.