On the master equation approach to diffusive grain-surface chemistry: The H, O, CO system

Stantcheva, Tatiana; Shematovich, V. I.; Herbst, Eric

doi:10.1051/0004-6361:20020838

Cited by 81 publications

(115 citation statements)

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“…Our 8.0 K data lie on the high end of the results, probably for two reasons. Our n(CO) is relatively high throughout the entire simulation, although it should be comparable to the intermediate abundance of Stantcheva et al (2002), and our limited network might overproduce H 2 CO and CH 3 OH instead of forming H 2 O and CO 2 for instance. Our simulations are however geared towards conditions where most of the atomic oxygen is locked up in CO, and H and CO are indeed the most important reactants on the surface.…”

Section: Comparison With Other Surface Modelsmentioning

confidence: 86%

“…Both chemistries were treated using rate equations for four different scenarios defined by slow and fast diffusion rates combined with atomic and molecular initial conditions. The circles represent master equation results from Stantcheva et al (2002). Here for a wide range of densities (10 3 , 10 4 , 10 5 cm −3 ) the gas-grain chemistry for a limited set of surface reactions is obtained.…”

Section: Comparison With Other Surface Modelsmentioning

confidence: 99%

“…9 represent data from Chang et al (2007) obtained by a similar Monte Carlo method to that used in the present paper (Chang et al 2005), which includes both the layering effect as well as the competition of the reaction barriers. The surface chemistry is coupled to the gas phase chemistry and the surface chemistry network is very similar to one used by Stantcheva et al (2002). The square symbols represent results for n H = 2 × 10 4 cm −3 and a surface temperature of 10 and 15 K. The main difference between the method by Chang et al (2007) and the present simulations is that our Monte Carlo algorithm is optimised to reproduce laboratory experiments and it includes important features of the CO + H system that were revealed through this optimisation.…”

Section: Comparison With Other Surface Modelsmentioning

confidence: 99%

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Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Cuppen

Dishoeck

Herbst

et al. 2009

A&A

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Context. Methanol and its precursor formaldehyde are among the most studied organic molecules in the interstellar medium and are abundant in the gaseous and solid phases. We recently developed a model to simulate CO hydrogenation via H atoms on interstellar ice surfaces, the most important interstellar route to H 2 CO and CH 3 OH, under laboratory conditions. Aims. We extend this model to simulate the formation of both organic species under interstellar conditions, including freeze-out from the gas and hydrogenation on surfaces. Our aim is to compare calculated abundance ratios with observed values and with the results of prior models. Methods. Our model utilises the continuous-time, random-walk Monte Carlo method, which -unlike other approaches -is able to simulate microscopic grain-surface chemistry over the long timescales in interstellar space, including the layering of ices during freeze-out. Results. Simulations under different conditions, including density and temperature, have been performed. We find that H 2 CO and CH 3 OH form efficiently in cold dense cores or the cold outer envelopes of young stellar objects. The grain mantle is found to have a layered structure with CH 3 OH on top. The species CO and H 2 CO are found to exist predominantly in the lower layers of ice mantles where they are not available for hydrogenation at late times. This finding is in contrast with previous gas-grain models, which do not take into account the layering of the ice. Some of our results can be reproduced by a simple quasi-steady-state analytical model that focuses on the outer layer. Conclusions. Observational solid H 2 CO/CH 3 OH and CO/CH 3 OH abundance ratios in the outer envelopes of an assortment of young stellar objects agree reasonably well with our model results, which also suggest that the large range in CH 3 OH/H 2 O observed abundance ratios is due to variations in the evolutionary stages. Finally, we conclude that the limited chemical network used here for surface reactions apparently does not alter the overall conclusions.

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Section: Comparison With Other Surface Modelsmentioning

confidence: 86%

Section: Comparison With Other Surface Modelsmentioning

confidence: 99%

Section: Comparison With Other Surface Modelsmentioning

confidence: 99%

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Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Cuppen

Dishoeck

Herbst

et al. 2009

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“…Secondly, there must be a mechanism to release methanol formed on grains into the gas phase, which is also not easy to accomplish under these conditions, since the desorption energy of methanol is comparatively high. 38 Gas-grain models have included repetitive hydrogenation of CO leading to methanol. 39 Unfortunately, two of the steps involved, namely the reactions H + CO -HCO and H + H 2 CO -H 3 CO are not barrierless and exhibit thresholds of 26.4 and 73.6 meV, respectively.…”

Section: Astrophysical Implicationsmentioning

confidence: 99%

Dissociative recombination of protonated methanol

et al. 2006

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The branching ratios of the different reaction pathways and the overall rate coefficients of the dissociative recombination reactions of CH 3 OH 2 + and CD 3 OD 2 + have been measured at the CRYRING storage ring located in Stockholm, Sweden. Analysis of the data yielded the result that formation of methanol or deuterated methanol accounted for only 3 and 6% of the total rate in CH 3 OH 2 + and CD 3 OD 2 + , respectively. Dissociative recombination of both isotopomeres mainly involves fragmentation of the C-O bond, the major process being the three-body break-up forming CH 3 , OH and H (CD 3 , OD and D). The overall cross sections are best fitted by s = 1.2 AE 0.1 Â 10 À15 E À1.15AE0.02 cm 2 and s = 9.6 AE 0.9 Â 10 À16 E À1.20AE0.02 cm 2 for CH 3 OH 2 + and CD 3 OD 2 + , respectively. From these values thermal reaction rate coefficients of k(T) = 8.9 AE 0.9 Â 10 À7 (T/300) À0.59AE0.02 cm 3 s À1 (CH 3 OH 2 + ) and k(T) = 9.1 AE 0.9 Â 10 À7 (T/300) À0.63AE0.02 cm 3 s À1 (CD 3 OD 2 + ) can be calculated. A non-negligible formation of interstellar methanol by the previously proposed mechanism via radiative association of CH 3 + and H 2 O and subsequent dissociative recombination of the resulting CH 3 OH 2 + ion to yield methanol and hydrogen atoms is therefore very unlikely.

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“…Thus, we have two sets of accretion rates. Number densities are taken from Stantcheva et al (2002). These are listed in Table 1.…”

Section: Accretion Ratementioning

confidence: 99%

Formation of water and methanol in star forming molecular clouds

Das¹,

Acharyya

Chakrabarti³

2008

A&A

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Aims. We study the formation of water and methanol in the dense cloud conditions to find the dependence of its production rate on the binding energies, reaction mechanisms, temperatures, and grain site number. We wish to find the effective grain surface area available for chemical reaction and the effective recombination timescales as functions of grain and gas parameters. Methods. We used a Monte Carlo simulation to follow the chemical processes occurring on the grain surface. We carried out the simulations on the Olivine grains of different sizes, temperatures, gas phase abundances and different reaction mechanisms. We consider H, O, and CO as the accreting species from the gas phase and allow ten chemical reactions among them on the grains. Results. We find that the formation rate of various molecules is strongly dependent on the binding energies. When the binding energies are high, it is very difficult to produce significant amounts of the molecular species. Instead, the grain is found to be full of atomic species. The production rates are found to depend on the number density in the gas phase. When the density is high, the production of various molecules on the grains is small as grain sites are quickly filled up by atomic species. If both the Eley-Rideal and Langmuir-Hinselwood mechanisms are considered, then the production rates are maximum and the grains are filled up relatively faster. Thus, if allowed, the Eley-Rideal mechanism can also play a major role and more so when the grain is full of immobile species. We show that the concept of the effective grain surface area, which we introduced in our earlier work, plays a significant role in grain chemistry. Conclusions. We compute the abundance of water and methanol and show that the results strongly depend on the density and composition in the gas phase, as well as various grain parameters. In the rate equation, it is generally assumed that the recombination efficiencies are independent of the grain parameters, and the surface coverage. Presently, our computed parameter α for each product is found to depend on the accretion rate, the grain parameters and the surface coverage of the grain. We compare our results obtained from the rate equation and the one from the effective rate equation, which includes α. A comparison of our results with the observed abundance shows very good agreement.

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On the master equation approach to diffusive grain-surface chemistry: The H, O, CO system

Cited by 81 publications

References 24 publications

Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores

Dissociative recombination of protonated methanol

Formation of water and methanol in star forming molecular clouds

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