New gas–grain chemical models of quiescent dense interstellar clouds: the effects of H2 tunnelling reactions and cosmic ray induced desorption

Abstract

New models of the chemistry of dense interstellar clouds are presented in which both gas-phase and grain-surface chemistry occur. The dust-grain and gas temperatures are fixed at 10 K and the gas density n = n(H) + 2n(H₂) remains approximately at 2 × 10⁴ cm^–3 in these models, in order to represent the chemistry occurring in quiescent clouds. Our previous model has been improved in several substantial ways. The gas-phase network has been extended to 2671 reactions, mainly with the inclusion of recent results involving organo-sulphur reactions. The surface-chemistry network has been extended to 254 reactions. The most significant new surface reactions are exothermic processes of the general type X + H₂ → XH + H, where X is a molecule, and where a significant activation energy of uncertain magnitude exists. These reactions can occur at appreciable rates due to tunnelling if the activation energy barriers are sufficiently low. If they occur at appreciable rates, the reactions tend to quench complex molecule formation on dust particles. We have also developed simplified rate coefficients to represent cosmic ray induced desorption by impulsive heating of entire grains. Inclusion of this process in our models results in the preferential desorption of selected simple molecules from grain surfaces, but does not allow the desorption of complex organic molecules, which are bound more strongly to grain surfaces. Thus gas-phase observations of many interstellar molecules can still not be accounted for in our models at times later than ≈ 10⁶ yr.