Time-dependent H2 formation and protonation

TL;DR

This study models the time-dependent formation of molecular hydrogen and protonation in interstellar clouds, revealing that H2 formation timescales are long and largely unaffected by typical density variations, with implications for molecular cloud chemistry.

Contribution

The paper introduces a self-consistent numerical integration method for microscopic H2 formation and protonation equations, providing new insights into timescales and abundance patterns in diffuse interstellar gas.

Findings

H2 formation timescale is 10-30 million years in typical diffuse clouds

H3+ abundance is weakly dependent on cosmic-ray ionization rate at low H2 fractions

Late-time OH formation is prominent in diffuse regions with modest density and ionization

Abstract

Methods: 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. Results: The equilibrium H2 formation timescale in an H I cloud with N(H) ~ 4x10^{20}/cm^2 is 1-3 x 10^7 yr, nearly independent of the assumed density or H2 formation rate constant 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, formation of H2 is favored in larger regions having moderate density, consistent with the rather high mean kinetic temperatures measured in H2, 70-80 K. 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.