Computational Modeling of High Temperature Gas-Grain Chemistry in Interstellar Medium (ISM)

Abstract:
We present a modified version of the Nautilus three-phase model in which high temperature gas-grain processes have been considered. 316 reactions have been added and 18 new species for a total of 11,800 reactions and 734 species. The modifications were necessary to account for the new gas-grain processes added where chemistry is dominated by species bound in chemisorption sites. A new thermal dissociation process was added which is believed to be the dominant dissociation process at high temperatures. In this model the mantle is considered chemically inactive since the grain is believed to be relatively bare in these temperature regimes. We focused on studying carbonaceous grains and assume the grain has catalytic properties similar to a graphite surface. Gas-grain formation routes were responsible for the gas phase abundances of methane, molecular hydrogen and the hydroxyl radical (from 200-400 K). A new dissociative adsorption mechanism is introduced for molecular hydrogen which provides an efficient destruction route at T ≥ 500 K which results in a decrease in fractional abundance of up to two orders of magnitude when compared with the previous model. This reduction in molecular hydrogen results in the abundance of water being reduced when compared with previous models. An overall enhancement of atomic oxygen in the gas phase by almost an order of magnitude from T ≥ 500 K is observed and alkanes are therefor destroyed more efficiently, especially at high temperatures.