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A New and Simple Approach to Determine the Abundance of Hydrogen Molecules on Interstellar Ice Mantles | | Ugo Hincelin
; Qiang Chang
; Eric Herbst
; | | Date: |
27 Oct 2014 | | 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. | | Source: | arXiv, 1410.7375 | | Services: | Forum | Review | PDF | Favorites | |
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