A New and Simple Approach to Determine the Abundance of Hydrogen Molecules on Interstellar Ice Mantles

Abstract

Water is usually the main component of ice mantles, which cover the cores of dust grains in cold portions of dense interstellar clouds. When molecular hydrogen is adsorbed onto an icy mantle through physisorption, a common assumption in gas-grain rate equation models is to use an adsorption energy for molecular hydrogen on a pure water substrate. However, at high density and low temperature, when H2 is efficiently adsorbed onto the mantle, its surface abundance can be strongly overestimated if this assumption is still used. Unfortunately, the more detailed microscopic Monte Carlo treatment cannot be used to study the abundance of H2 in ice mantles if a full gas-grain network is utilized. We present a numerical method adapted for rate-equation models that takes into account the possibility that an H2 molecule can, while diffusing on the surface, find itself bound to another hydrogen molecule, with a far weaker bond than the H2-water bond, which can lead to more efficient desorption. We label the ensuing desorption "encounter desorption". The method is implemented first in a simple system consisting only of hydrogen molecules at steady state between gas and dust using the rate-equation approach and comparing the results with the results of a microscopic Monte Carlo calculation. We then discuss the use of the rate-equation approach with encounter desorption embedded in a complete gas-grain chemical network. For both systems, the rate-equation model with encounter desorption reproduces the H2 granular coverage computed by the microscopic Monte Carlo model. The method is especially useful for dense and cold environments, and for time-dependent physical conditions, such as occur in the collapse of dense cores and the formation of protoplanetary disks. It is not significantly CPU time consuming, so can be used for example with complex 3D chemical-hydrodynamical simulations.

8 pages, 7 figures, language editing, accepted for publication in Astronomy & Astrophysics

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• A new model of the chemistry of ionizing radiation in solids: CIRIS
• Chemical complexity of phosphorous bearing species in various regions of the Interstellar medium
• Is there any linkage between interstellar aldehyde and alcohol?
• Importance of the H2 abundance in protoplanetary disk ices for the molecular layer chemical composition
• The KOSMA-$\tau$ PDR Model -- I. Recent updates to the numerical model of photo-dissociated regions
• The connection between warm carbon chain chemistry and interstellar irradiation of star-forming cores
• Effect of binding energies on the encounter desorption
• Astrochemical models of interstellar ices: History matters
• A multigrain-multilayer astrochemical model with variable desorption energy for surface species
• Effect of surface H${}_2$ on molecular hydrogen formation on interstellar grains
• A sensitivity analysis of interstellar ice chemistry in astrochemical models