Grain Surface Chemistry: Modified Models

Shalabiea, Osama M.; Caselli, P.; Herbst, Eric

doi:10.1086/305942

Cited by 56 publications

(69 citation statements)

References 15 publications

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“…The pseudo-time-dependent model evolves under these fixed physical conditions where the chemical abundances are allowed to evolve from the initial values until steady state is reached. The enhanced gas-grain network from the so-called unmodified model (Shalabiea et al 1998) has been used based on the earlier models of Hasegawa & Herbst (1993) and Hasegawa et al (1992). It contains 656 species and 4742 reactions.…”

Section: The Chemical Models and Initial Conditionsmentioning

confidence: 99%

“…Many of the key rates of reactions have been improved in the gas-phase. Also gas-grain interactions have been given more attention, showing their crucial role in chemical models (Hasegawa et al 1992;Willacy & Williams 1993;Shalabiea & Greenberg 1994;Shalabiea et al 1998;Willacy & Millar 1998;Ruffle & Herbst 2000, 2001. However after years of extensive theoretical, experimental, and observational studies, interstellar chemical modeling still has many puzzling problems.…”

Section: Introductionmentioning

confidence: 99%

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The effect of the initial elemental abundance on gas-grain chemical models

Shalabiea

2001

A&A

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Abstract. For any chemical modeling, it is important to recognize that the adopted set of initial elemental abundances is a crucial parameter. The effect of initial abundance variation has been investigated. Using the most recent observations and theoretical grain models, we have set some constraints upon the set of the initial elemental abundances. Both gas-phase and gas-grain chemical models are used in this study. At early-time stages less than 1 Myr, there is little difference between results with different initial [C]/[O] ratios. This holds for gas-phase and gas-grain models. At a later evolutionary time or in the steady state, the result of the gas-grain model shows little or no dependence on the initial [C]/[O] ratios. By contrast, at late or steady-state times, the abundances of chemical species using gas-phase models are very sensitive to any variation of the initial [C]/[O] ratios. Sulfur depletion is needed for both gas-phase and gas-grain models to reproduce the observed sulfur-bearing molecules. Our main conclusion is that the gas-grain interaction processes such as accretion, surface reactions, and desorption minimize the vital role of the initial set of elemental abundance in gas-grain chemical models.

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Section: The Chemical Models and Initial Conditionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The effect of the initial elemental abundance on gas-grain chemical models

Shalabiea

2001

A&A

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“…If a grain surface is expected to contain few reactive species, such an approach to the treatment of the surface chemistry provides more accurate results than the deterministic rate equation method (Ruffle & Herbst 2000). Owing to the computational expense of Monte Carlo methods, and the fact that, to date, it has not been possible to couple such an approach with a fully time-dependent gas-phase chemistry, Caselli et al (1998) and Shalabiea et al (1998) have introduced and employed modifications to the deterministic rate equations in an attempt to generalize such models so that they can be applied validly to problems in which the surface chemistry is accretion limited.…”

Section: Introductionmentioning

confidence: 99%

A stochastic approach to grain surface chemical kinetics

Green

Toniazzo

Pilling

et al. 2001

A&A

148

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Abstract. A stochastic model of grain surface chemistry, based on a master equation description of the probability distributions of reactive species on grains, is developed. For an important range of conditions, rates of molecule formation are limited by low accretion rates, so that the probability that a grain contains more than one reactive atom or molecule is small. We derive simple approximate expressions for these circumstances, and explore their validity through comparison with numerical solutions of the master equation for H, O and H, N, O reaction systems. A more detailed analysis of the range of validity of several analytic approximations and numerical solutions, based on exact analytical results for a model in which H and H2 are the only species, is also made. Though the use of our simple approximate expressions is computationally efficient, the solution of the master equation under the assumption that no grain contains more than two particles of each species usually gives more accurate results in the parameter regimes where the deterministic rate equation approach is inappropriate. The implementation of sparse matrix inversion techniques makes the use of such a truncated master equation solution method feasible for considerably more complicated surface chemistries than the ones we have examined here.

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“…Therefore, rate equations are not suitable [37,38,39,40] and stochastic methods such as the direct integration of the master equation [12,41] or MC simulations [42] are required. As discussed above, these methods apply in the case of small networks but become infeasible for large networks [43,44].…”

Section: Introductionmentioning

confidence: 99%

Efficient stochastic simulations of complex reaction networks on surfaces

Barzel

Biham

2007

The Journal of Chemical Physics

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Surfaces serve as highly efficient catalysts for a vast variety of chemical reactions. Typically, such surface reactions involve billions of molecules which diffuse and react over macroscopic areas.Therefore, stochastic fluctuations are negligible and the reaction rates can be evaluated using rate equations, which are based on the mean-field approximation. However, in case that the surface is partitioned into a large number of disconnected microscopic domains, the number of reactants in each domain becomes small and it strongly fluctuates. This is, in fact, the situation in the interstellar medium, where some crucial reactions take place on the surfaces of microscopic dust grains. In this case rate equations fail and the simulation of surface reactions requires stochastic methods such as the master equation. However, in the case of complex reaction networks, the master equation becomes infeasible because the number of equations proliferates exponentially.To solve this problem, we introduce a stochastic method based on moment equations. In this method the number of equations is dramatically reduced to just one equation for each reactive species and one equation for each reaction. Moreover, the equations can be easily constructed using a diagrammatic approach. We demonstrate the method for a set of astrophysically relevant networks of increasing complexity. It is expected to be applicable in many other contexts in which problems that exhibit analogous structure appear, such as surface catalysis in nanoscale systems, aerosol chemistry in stratospheric clouds and genetic networks in cells.

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Grain Surface Chemistry: Modified Models

Cited by 56 publications

References 15 publications

The effect of the initial elemental abundance on gas-grain chemical models

The effect of the initial elemental abundance on gas-grain chemical models

A stochastic approach to grain surface chemical kinetics

Efficient stochastic simulations of complex reaction networks on surfaces

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