674 research outputs found
Master Equation for Hydrogen Recombination on Grain Surfaces
Recent experimental results on the formation of molecular hydrogen on
astrophysically relevant surfaces under conditions similar to those encountered
in the interstellar medium provided useful quantitative information about these
processes. Rate equation analysis of experiments on olivine and amorphous
carbon surfaces provided the activation energy barriers for the diffusion and
desorption processes relevant to hydrogen recombination on these surfaces.
However, the suitability of rate equations for the simulation of hydrogen
recombination on interstellar grains, where there might be very few atoms on a
grain at any given time, has been questioned. To resolve this problem, we
introduce a master equation that takes into account both the discrete nature of
the H atoms and the fluctuations in the number of atoms on a grain. The
hydrogen recombination rate on microscopic grains, as a function of grain size
and temperature, is then calculated using the master equation. The results are
compared to those obtained from the rate equations and the conditions under
which the master equation is required are identified.Comment: Latex document. 14 pages of text. Four associated figs in in PS
format on separate files that are "called-in" the LaTeX documen
Formation of molecular hydrogen on analogues of interstellar dust grains: experiments and modelling
Molecular hydrogen has an important role in the early stages of star
formation as well as in the production of many other molecules that have been
detected in the interstellar medium. In this review we show that it is now
possible to study the formation of molecular hydrogen in simulated
astrophysical environments. Since the formation of molecular hydrogen is
believed to take place on dust grains, we show that surface science techniques
such as thermal desorption and time-of-flight can be used to measure the
recombination efficiency, the kinetics of reaction and the dynamics of
desorption. The analysis of the experimental results using rate equations gives
useful insight on the mechanisms of reaction and yields values of parameters
that are used in theoretical models of interstellar cloud chemistry.Comment: 23 pages, 7 figs. Published in the J. Phys.: Conf. Se
Molecular Hydrogen Formation on Ice Under Interstellar Conditions
The results of experiments on the formation of molecular hydrogen on low
density and high density amorphous ice surfaces are analyzed using a rate
equation model. The activation energy barriers for the relevant diffusion and
desorption processes are obtained. The more porous morphology of the low
density ice gives rise to a broader spectrum of energy barriers compared to the
high density ice. Inserting these parameters into the rate equation model under
steady state conditions we evaluate the production rate of molecular hydrogen
on ice-coated interstellar dust grains.Comment: 20 pages, 3 tables and 10 figures. Accepted to ApJ. Minor changes
made and adittional references adde
A Unified Monte Carlo Treatment of Gas-Grain Chemistry for Large Reaction Networks. I. Testing Validity of Rate Equations in Molecular Clouds
In this study we demonstrate for the first time that the unified Monte Carlo
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.
We found that at temperatures 25--30 K gas-phase abundances of HO,
NH, 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 3x 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 Monte Carlo and deterministic calculations in the same temperature range. At
lower temperatures ( K) gaseous and, in particular, surface abundances
of most important molecules are not much affected by stochastic processes.Comment: 33 pages, 9 figures, 1 table. Accepted for publication in Ap
The effect of grain size distribution on H formation rate in the interstellar medium
The formation of molecular hydrogen in the interstellar medium takes place on
the surfaces of dust grains. Hydrogen molecules play a role in gas-phase
reactions that produce other molecules, some of which serve as coolants during
gravitational collapse and star formation. Thus, the evaluation of the
roduction rate of hydrogen molecules and its dependence on the physical
conditions in the cloud are of great importance. Interstellar dust grains
exhibit a broad size distribution in which the small grains capture most of the
surface area. Recent studies have shown that the production efficiency strongly
depends on the grain composition and temperature as well as on its size. In
this paper we present a formula which provides the total production rate of
H per unit volume in the cloud, taking into account the grain composition
and temperature as well as the grain size distribution. The formula agrees very
well with the master equation results. It shows that for a physically relevant
range of grain temperatures, the production rate of H is significantly
enhanced due to their broad size distribution.Comment: to appear in MNRA
H2 Formation on Interstellar Grains in Different Physical Regimes
An analysis of the kinetics of H2 formation on interstellar dust grains is
presented using rate equations. It is shown that semi-empirical expressions
that appeared in the literature represent two different physical regimes. In
particular, it is shown that the expression given by Hollenbach, Werner and
Salpeter [ApJ, 163, 165 (1971)] applies when high flux, or high mobility, of H
atoms on the surface of a grain, makes it very unlikely that H atoms evaporate
before they meet each other and recombine. The expression of Pirronello et al.\
[ApJ, 483, L131 (1997)] -- deduced on the basis of accurate measurements on
realistic dust analogue -- applies to the opposite regime (low coverage and low
mobility). The implications of this analysis for the understanding of the
processes dominating in the Interstellar Medium are discussed.Comment: 4 pages, MN styl
Moment equations for chemical reactions on interstellar dust grains
While most chemical reactions in the interstellar medium take place in the
gas phase, those occurring on the surfaces of dust grains play an essential
role. Chemical models based on rate equations including both gas phase and
grain surface reactions have been used in order to simulate the formation of
chemical complexity in interstellar clouds. For reactions in the gas phase and
on large grains, rate equations, which are highly efficient to simulate, are an
ideal tool. However, for small grains under low flux, the typical number of
atoms or molecules of certain reactive species on a grain may go down to order
one or less. In this case the discrete nature of the opulations of reactive
species as well as the fluctuations become dominant, thus the mean-field
approximation on which the rate equations are based does not apply. Recently, a
master equation approach, that provides a good description of chemical
reactions on interstellar dust grains, was proposed. Here we present a related
approach based on moment equations that can be obtained from the master
equation. These equations describe the time evolution of the moments of the
distribution of the population of the various chemical species on the grain. An
advantage of this approach is the fact that the production rates of molecular
species are expressed directly in terms of these moments. Here we use the
moment equations to calculate the rate of molecular hydrogen formation on small
grains. It is shown that the moment equation approach is efficient in this case
in which only a single reactive specie is involved. The set of equations for
the case of two species is presented and the difficulties in implementing this
approach for complex reaction networks involving multiple species are
discussed.Comment: 12 pages, submitted for publication in A&
On the master equation approach to diffusive grain-surface chemistry: the H, O, CO system
We have used the master equation approach to study a moderately complex
network of diffusive reactions occurring on the surfaces of interstellar dust
particles. This network is meant to apply to dense clouds in which a large
portion of the gas-phase carbon has already been converted to carbon monoxide.
Hydrogen atoms, oxygen atoms, and CO molecules are allowed to accrete onto dust
particles and their chemistry is followed. The stable molecules produced are
oxygen, hydrogen, water, carbon dioxide (CO2), formaldehyde (H2CO), and
methanol (CH3OH). The surface abundances calculated via the master equation
approach are in good agreement with those obtained via a Monte Carlo method but
can differ considerably from those obtained with standard rate equations.Comment: 13 pages, 5 figure
Enhanced production of HD and D_2 molecules on small dust grains in diffuse clouds
Motivated by recent observations of deuterated molecules in the interstellar
medium, we examine the production of HD and D molecules on dust grain
surfaces. A mechanism for the enhanced production of these deuterated molecules
is studied. This mechanism applies on grain surfaces on which D atoms stick
more strongly than H atoms, under conditions of low flux and within a suitable
range of temperatures. It is shown that under these conditions the production
rates of HD and D are greatly enhanced (vs. the H production rate)
compared with the expected rates based on the adsorption of gas-phase atomic
abundances of D and H. The enhancement in the formation rate of HD is
comparable with the enhancement due to gas-phase ion-molecule reactions in
diffuse clouds.Comment: This is a preprint of an article accepted for publication in Monthly
Notices of The Royal Astromnomical Societ
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