2007A&A...461..205L

To demonstrate the time-approach to equilibrium of H₂-formation and protonation in models of diffuse or HI interstellar gas clouds previously published by the author. The microscopic equations of H₂-formation and protonation are integrated numerically over time in such a manner that the overall structures evolve self-consistently under benign conditions. The equilibrium H₂ formation timescale in an H I cloud with N(H)≃4x10²⁰cm^–2 is 1-3x10⁷yr, nearly independent of the assumed density or H₂ formation rate on grains, etc. Attempts to speed up the evolution of the H₂-fraction would require densities well beyond the range usually considered typical of diffuse gas. The calculations suggest that, under benign, quiescent conditions, H₂ in the diffuse ISM formation of H₂ is favored in larger regions having moderate density, consistent with the rather high mean kinetic temperatures measured in H₂, 70-80K. Formation of H₃⁺ is essentially complete when H₂-formation equilibrates but the final abundance of H₃⁺ appears more nearly at the very last instant. Chemistry in a weakly-molecular gas has particular properties so that the abundance patterns change appreciably as gas becomes more fully molecular, either in model sequences or with time in a single model. One manifestation of this is that the predicted abundance of H₃⁺ is much more weakly dependent on the cosmic-ray ionization rate when n(H₂)/n(H) ≲0.05. In general, high abundances of H₃⁺ do not enhance the abundances of other species (e.g. HCO⁺) but late-time OH formation proceeds most vigourously in more diffuse regions having modest density, extinction and H₂ fraction and somewhat higher fractional ionization, suggesting that atypically high OH/H₂ abundance ratios might be found optically in diffuse clouds having modest extinction.