Continuous-time random-walk simulation of H$\mathsf{_{2}}$ formation on interstellar grains

Abstract

Abstract. The formation of molecular hydrogen via the recombination of hydrogen atoms on interstellar grain surfaces has been investigated anew. A detailed Monte Carlo procedure known as the continuous-time random-walk method has been used. This Monte Carlo approach has two advantages over the stochastic master equation method: it treats random walk on a surface correctly, and it can easily be used for inhomogeneous surfaces. The recombination efficiency for H 2 formation as a function of surface temperature and grain size has been calculated for a variety of grain surfaces with a flux of hydrogen atoms representative of diffuse interstellar clouds. The surfaces studied include homogeneous olivine and amorphous carbon, characterized by single energies for the diffusion barrier and binding energy of H atoms, inhomogeneous versions of these two surfaces with distributions of H-atom diffusion barriers and binding energies, and a variety of mixed surfaces of olivine and carbon. For the homogeneous surfaces, we confirm that the temperature range for efficient formation of H 2 is very small. At temperatures near peak efficiency, there is little dependence on grain size. At temperatures higher than those of peak efficiency, the Monte Carlo procedure exhibits smaller efficiencies for molecular hydrogen formation than the master equation method in the limit of large grain sizes. For various types of inhomogeneous and mixed surfaces, the major effect we find is an increase in the temperature range over which the efficiency of molecular hydrogen formation is high. Efficient formation of H 2 in diffuse interstellar clouds now seems possible with inhomogeneous grains.

The chemistry of high-mass star formation

“…The composition of the ice mantle depends primarily on the atomic composition of the gas phase, with a weaker dependence on grain temperature, through differences in the volatility of atomic species (Tielens & Hagen 1982). Recent simulations (Chang et al 2005) and lab experiments (Hornekaer et al 2003) show that the nature of the surface also plays an important role.…”

Formation of alcohols on ice surfaces

“…In contrast with rate equation methods that are often applied, the method follows the individual atoms during a simulation, in this way one can consider the layering and topology of the system. For a detailed description of the Formation of alcohols on ice surfaces 379 method we refer to previous papers (Chang et al 2005, Cuppen & Herbst 2007.…”

Deuterium enrichment of the interstellar grain mantle

“…Reason for choosing 50 × 50 site grains is that it is well above the limit of the statistical fluctuation (Chang, Cuppen & Herbst 2005). Here, all the events are processed by generating random numbers.…”

“…Mainly silicate and carbonaceous grains dominate in an interstellar space and they are thought to have molecular ice layers (Draine 2003;Gibb et al 2004). In fact, even the huge abundance of molecular hydrogen could not be explained without invoking interstellar dusts (Biham et al 2001;Chang, Cuppen & Herbst 2005;Chakrabarti et al 2006a,b;Sahu et al 2015). It is now believed that at least some significant fraction of the interstellar species would be synthesized on interstellar dusts ( Stars are mainly formed in the densest part of an interstellar cloud called as pre-stellar core.…”