Molecular hydrogen formation during dense interstellar cloud collapse

Acharyya, Kinsuk; Chakrabarti, Sandip K.

doi:10.1111/j.1365-2966.2005.09195.x

Cited by 11 publications

(10 citation statements)

References 28 publications

(58 reference statements)

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“…It may be instructive to check if the rate equations are modified with r ab /S replaced by r ab /S α everywhere, and they are solved by the usual method (Acharyya et al 2005), then whether the results become comparable to those obtained from our Monte-Carlo method. In Fig. 14, we made a comparison of the abundances of two of the important species, namely, H 2 O and CH 3 OH in different methods.…”

Section: Comparison Of Results With the Effective Rate Equationsmentioning

confidence: 99%

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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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Section: Comparison Of Results With the Effective Rate Equationsmentioning

confidence: 99%

“…The rate equation method belongs to this category. This method is very extensively used by several authors to study the grain surface chemistry (Hasegawa & Herbst 1992;Roberts et al 2002;Acharyya et al 2005). However, this method is only applicable when there are large numbers of reactants on the grain surface.…”

Section: Introductionmentioning

confidence: 99%

Formation of water and methanol in star forming molecular clouds

Das¹,

Acharyya

Chakrabarti³

2008

A&A

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“…The dust provides a surface for hydrogen atoms to meet and react and also removes enough of the reaction exothermicity so as to stabilize H 2 formation. In the last few decades, the formation of H 2 in the ISM has been studied in extensive detail, both theoretically (Gould & Salpeter 1963;Hollenbach & Salpeter 1970Smoluchowski 1981Smoluchowski , 1983Aronowitz & Chang 1985;Duley & Williams 1986;Farebrother et al 1999;Biham et al 2001;Green et al 2001;Chang et al 2005Chang et al , 2006Acharyya et al 2005;Chakrabarti et al 2006) and experimentally (Brackmann & Fite 1961;Schutte et al 1976;Pirronello & Averna 1988;Pirronello et al 1997aPirronello et al , 1997bPirronello et al , 1999Manicò et al 2001;Hornekaer et al 2003;Roser et al 2002Roser et al , 2003.…”

Section: Introductionmentioning

confidence: 99%

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Iqbal

Acharyya

Herbst

2012

ApJ

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We have used the continuous-time random-walk Monte Carlo technique to study the formation of H 2 from two hydrogen atoms on the surface of interstellar dust grains with both physisorption and chemisorption sites on olivine and carbonaceous material. In our standard approach, atoms must first enter the physisorption site before chemisorption can occur. We have considered hydrogen atom mobility due to both thermal hopping and quantum mechanical tunneling. The temperature range between 5 K and 825 K has been explored for different incoming H fluxes representative of interstellar environments with atomic hydrogen number density ranging between 0.1 cm −3 and 100 cm −3 and dust grain sizes ranging from 100 sites to 10 6 sites, the latter corresponding roughly to olivine grains of radius 0.2 μm. In addition, we have also considered rough surfaces with multiple binding sites. Tunneling is found to dominate the surface chemistry at low temperature, but as the temperature increases, the scenario changes. The inclusion of chemisorption sites can provide a meaningful efficiency for H 2 production up to temperatures as high as 700 K depending upon the depth of the chemisorption well. We found that over virtually the entire temperature range studied, the use of rate equations overestimates the H 2 formation rate to some extent. This overestimate is large at high temperatures, due to very low surface residence times. We have also considered models in which chemisorption sites are entered directly and diffusion proceeds only to other chemisorption sites.

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“…However, in their initial approach to the time-dependent modelling, they assumed a constant temperature but allowed only the density to vary. Ceccarelli et al (1996) used the "inside-out", isothermal, spherical collapse model of Shu In this backdrop of this observational and experimental status, we carry out our investigation by improving earlier work by first incorporating the accurate grain chemistry as elaborated in Acharyya et al (2005) and then by actually combining the results of a time dependent hydrodynamics code with the chemical evolution code to see how the abundances vary with the grid locations. We also chose initial conditions very much different from the earlier studies.…”

Section: Introductionmentioning

confidence: 99%

Time evolution of simple molecules during proto-star collapse

et al. 2008

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We study the formation and evolution of several molecules in a collapsing interstellar cloud using a reasonably large reaction network containing more then four hundred atomic and molecular species. We employ a time dependent, spherically symmetric, hydrodynamics code to follow the hydrodynamic and chemical evolution of the collapsing cloud. The flow is assumed to be self-gravitating. We use two models to study the hydrodynamic evolution: in the first model, we inject matter into an initially low density 2 Das et al.region and in the second model, we start with a constant density cloud and let it collapse due to self-gravity. We study the evolution of the central core for both the cases. We include the grain chemistry to compute the formation of molecular hydrogen and carried out the effect of gas and grain chemistry at each time step. We follow the collapse for more than 1014 s (about 3 million years) and present the time evolution of the globally averaged abundances of various simple but biologically important molecules, such as glycine, alanine etc. We compare our results with those obtained from observations found that for lighter molecules the agreement is generally very good. For complex molecules we tend to under predict the abundances. This indicates that other pathways could be present to form these molecules or more accurate reaction rates were needed.

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Molecular hydrogen formation during dense interstellar cloud collapse

Cited by 11 publications

References 28 publications

Formation of water and methanol in star forming molecular clouds

Formation of water and methanol in star forming molecular clouds

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Time evolution of simple molecules during proto-star collapse

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Molecular hydrogen formation during dense interstellar cloud collapse

Cited by 11 publications

References 28 publications

Formation of water and methanol in star forming molecular clouds

Formation of water and methanol in star forming molecular clouds

KINETIC MONTE CARLO STUDIES OF H2FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

Time evolution of simple molecules during proto-star collapse

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About

KINETIC MONTE CARLO STUDIES OF H₂FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE