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A Common Gas-Phase Chemistry for Diffuse, Translucent, and Dense Clouds?

This paper consolidates and extends the results of a large observational and chemical modeling study of translucent clouds. Thirty-eight molecular species have been observed in an ensemble of 38 translucent objects, for which detailed self-consistent physical models have been constructed. These are used with microturbulent radiative transfer analyses to obtain reliable fractional abundances. The abundances are essentially the same in translucent clouds and cold dense clouds (TMC-1). We have also scaled the column densities of the 12 species studied by Liszt & Lucas in diffuse clouds by adsorption against point-continuum sources to yield fractional abundances. These in turn are found to be remarkably similar to those in translucent and dense quiescent clouds. Thus a global abundance pattern emerges, which holds over a range of hundredfold in density, and transcends the completely photon-dominated chemistry regime to the collision-dominated regime. We have developed comprehensive quiescent gas-phase chemical models, based on the New Standard Model (NSM) reaction data set. We have found a single set of parameters (elemental abundances, depletion factors, and other physical conditions) that can explain the abundances of 34 of the 38 species observed in translucent clouds, spanning the transition region 1 ≤ Av0 ≤ 5 mag. The NSM is insensitive to density in the translucent region. The similarity of abundances in translucent and dense clouds (TMC-1) is explained by the same chemical model with a slightly higher depletion factor for C and O. To explain the diffuse cloud abundances, we need to augment the NSM reaction set of 4,300 reactions with just three turbulence-driven, weakly endothermic reactions as analyzed by Spaans. These latter reactions lose effect as the density increases from the diffuse to the translucent regime, in just such a way that the diffuse-cloud abundances seamlessly join with the translucent abundances, thus replicating the constancy of abundances from diffuse conditions (200 cm-3) to dense cloud conditions (20,000 cm-3). It seems clear that purely low-temperature gas-phase chemistry can explain many more observations than previously recognized.