2010ApJ...713..662A

The internal energy available in vibrationally excited H₂molecules can be used to overcome or diminish the activation barrier of various chemical reactions of interest for molecular astrophysics. In this paper, we investigate in detail the impact on the chemical composition of interstellar clouds of the reactions of vibrationally excited H₂ with C⁺, He⁺, O, OH, and CN, based on the available chemical kinetics data. It is found that the reaction of H₂(v>0) and C⁺ has a profound impact on the abundances of some molecules, especially CH⁺, which is a direct product and is readily formed in astronomical regions with fractional abundances of vibrationally excited H₂, relative to the ground state H₂, in excess of ∼10^–6, independently of whether the gas is hot or not. The effects of these reactions on the chemical composition of the diffuse clouds ζOph and HD 34078, the dense photon-dominated region (PDR) Orion Bar, the planetary nebula NGC 7027, and the circumstellar disk around the B9 star HD 176386 are investigated through PDR models. We find that formation of CH⁺ is especially favored in dense and highly FUV illuminated regions such as the Orion Bar and the planetary nebula NGC 7027, where column densities in excess of 10¹³/cm2 are predicted. In diffuse clouds, however, this mechanism is found to be not efficient enough to form CH⁺ with a column density close to the values derived from astronomical observations.