A unified Monte Carlo treatment of gas-grain chemistry for large reaction networks. I. Testing validity of rate equations in molecular clouds.
VASYUNIN A.I., SEMENOV D.A., WIEBE D.S. and HENNING Th.
Abstract (from CDS):
In this study, we demonstrate for the first time that the unified Monte Carlo (MC) approach can be applied to model gas-grain chemistry in large reaction networks. Specifically, we build a time-dependent gas-grain chemical model of the interstellar medium, involving about 6000 gas-phase and 200 grain-surface reactions. This model is used to test the validity of the standard and modified rate equation methods in models of dense and translucent molecular clouds and to specify under which conditions the use of the stochastic approach is desirable. Two cases are considered: (1) the surface mobility of all species is due to thermal hopping; (2) in addition to thermal hopping, a temperature-independent quantum tunneling for H and H2 is allowed. The physical conditions characteristic for the core and the outer region of the TMC1 cloud are adopted. The gas-phase rate file RATE 06 together with an extended set of gas-grain and surface reactions is utilized. We found that at temperatures of 25-30 K gas-phase abundances of H2O, NH3, CO, and many other gas-phase and surface species in the stochastic model differ from those in the deterministic models by more than an order of magnitude, at least when tunneling is accounted for and/or diffusion energies are three times lower than the binding energies. In this case, surface reactions, involving light species, proceed faster than accretion of the same species. In contrast, in the model without tunneling and with high binding energies, when the typical timescale of a surface recombination is greater than the timescale of accretion onto the grain, we obtain almost perfect agreement between results of MC and deterministic calculations in the same temperature range. At lower temperatures (∼10 K), gaseous and, in particular, surface abundances of most important molecules are not much affected by stochastic processes.
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04 38 42.0 +25 35 45
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Syntax of coordinates is : "ra dec (wtype) [error ellipse] quality bibcode" :
ra dec : right ascension and declination (unit and frame defined according to your Output Options)
Grey values are increasing the original precision due to the computation of frame transformations
(wtype) : wavelength class for the origin of the coordinates (Rad, mm, IR, Optical, UV, Xray, Gam)
[error ellipse] : measurement uncertainty, on (ra,dec) if the positional angle is 90 degrees, on (majaxis,minaxis) otherwise (in mas at defined epoch in the original catalogue),
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quality : flag of quality
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B : 0.01-0.1" + 2MASS, Tyc
A : VLBI, Hipparcos
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174.3852 -13.4312
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