Diffusive grain-surface chemistry involving the atoms and diatomic molecules of two elements

Abstract

Abstract.A model of the grain surface chemistry involving the accretion of atoms of two different elements, X and Y, and their reactions to form species X 2 , XY, and Y 2 was examined for a wide range of choices for the values of its three free parameters -the accretion rate of X and Y, the desorption rate of X and the grain surface sweeping time of Y, all considered relative to the grain surface sweeping rate of X. Relative production rates of the diatomics were calculated with five methods involving, respectively, a high-order truncation of the master equation, a low-order truncation of the master equation, the standard deterministic rate equation approach, a modified rate equation approach and a set of approximations which are in some cases appropriate for accretion dominated chemistry. The accuracies of the relative production rates calculated with the different methods were assessed for the wide range of model parameters. The more accurate of the low-truncation master equation calculations and the standard deterministic rate equation approach gives results which are in most cases within ten or twenty per cent of the results given by the high-truncation master equation calculations. For many cases, the more accurate of the low order truncation and the standard deterministic rate equation approaches is indicated by a consideration of the average number of atoms of the two species on the grain's surface.

H 2 formation on interstellar dust grains: The viewpoints of theory, experiments, models and observations

“…Approximations allowing faster solution have been proposed, such as the moment equation approach (Lipshtat and Biham, 2003;Le Petit et al, 2009). A careful comparison of the master equation methodology to various approximations of this approach, and to the simpler rate equation formalism, was presented in Rae et al (2003). The master equation approach has also been extended to simultaneously account for dust temperature fluctuations of small grains (Bron et al, 2014).…”

Effective grain surface area in the formation of molecular hydrogen in interstellar clouds

“…A quantitative estimation based on physisorption of the formation rate of H 2 was carried out by Biham et al (2001) and Green et al (2001). It was found that a significant production is possible in cooler (∼ 10 − 25K) clouds (Rae et al 2003). Cazaux & Tielens (2002, 2004 use both physisorption and chemisorption, to demonstrate that H 2 production is possible at high temperatures (∼ 200 − 400K) also.…”

Molecular hydrogen formation during dense interstellar cloud collapse

“…More recently, Biham et al (2001) and Green et al (2001) have computed H 2 production rate by physisorption. It was found that significant production is possible in cooler (∼10–25 K) clouds (see, also, Stantcheva, Shematovich & Herbst 2002; Rae et al 2003; Lipshtat, Biham & Biham 2004). Subsequently, Cazaux & Tielens (2002, 2004) used both physisorption and chemisorption, especially to demonstrate that H 2 production is possible at high temperatures (∼200–400 K), as well.…”