Molecular Hydrogen Formation on Ice Under Interstellar Conditions

Perets, Hagai B.; Biham, Ofer; Manicó, Giulio; Pirronello, V.; Roser, J. E.; Swords, Sol; Vidali, Gianfranco

doi:10.1086/430435

Cited by 96 publications

(161 citation statements)

References 38 publications

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“…The recombination reaction of hydrogen atoms, forming molecular hydrogen, is also taken into account. Since H 2 has a low binding energy with respect to water ice (Tielens & Allamandola 1987;Perets et al 2005), and since its reaction of formation releases chemical energy (Hollenbach & Salpeter 1970), we assumed that the H 2 molecules are desorbed in the gas phase as soon as they are formed. Finally, IR observations have shown that CO 2 may be a major component of ices with abundances reaching 30% with respect to to water in some cases (Dartois et al 1999;Pontoppidan et al 2008).…”

Section: Chemical Networkmentioning

confidence: 99%

Multilayer modeling of porous grain surface chemistry

Taquet

Ceccarelli

Kahane

2012

A&A

145

157

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Context. Mantles of iced water mixed with carbon monoxyde, formaldehyde, and methanol are formed during the so-called prestellar core phase. In addition, radicals are also thought to be formed on the grain surfaces, and to react to form complex organic molecules later on, during the so-called warm-up phase of the protostellar evolution. Aims. We aim to study the formation of the grain mantles during the prestellar core phase and the abundance of formaldehyde, methanol, and radicals trapped in them. Methods. We have developed a macrosopic statistic multilayer model that follows the formation of grain mantles with time and that includes two effects that may increase the number of radicals trapped in the mantles: i) during the mantle formation, only the surface layer is chemically active and not the entire bulk; and ii) the porous structure of grains allows the trapping reactive particles. The model considers a network of H, O, and CO forming neutral species such as water, CO, formaldehyde, and methanol, plus several radicals. We ran a large grid of models to study the impact of the mantle multilayer nature and grain porous structure. In addition, we explored how the uncertainty of other key parameters influences the mantle composition. Results. Our model predicts relatively high abundances of radicals, especially of HCO and CH 3 O (10 −9 −10 −7 ). In addition, the multilayer approach enables us to follow the chemical differentiation within the grain mantle, showing that the mantles are far from being uniform. For example, methanol is mostly present in the outer layers of the mantles, whereas CO and other reactive species are trapped in the inner layers. The overall mantle composition depends on the density and age of the prestellar core as well as on some microscopic parameters, such as the diffusion energy and the hydrogenation reactions activation energy. Comparison with observations allows us to constrain the value of the last two parameters (0.5-0.65 and 1500 K, respectively) and provide some indications on the physical conditions during the formation of the ices.

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Section: Chemical Networkmentioning

confidence: 99%

Multilayer modeling of porous grain surface chemistry

Taquet

Ceccarelli

Kahane

2012

A&A

145

157

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“…Note that in the analysis of the experimental results, presented in Katz et al (1999) and Perets et al (2005), no distinction between H and D atoms was made to ensure that the number of fitting parameters was small. The possibility of the tunneling of adsorbed atoms between adsorption sites was studied by Cazaux & Tielens (2004).…”

Section: The Interaction Between Hydrogen/deuterium Atoms and Dust Grmentioning

confidence: 99%

“…These parameters are based on a series of experiments and subsequent analysis, reported in Katz et al (1999), Perets et al (2005) and Perets et al (2007). The attempt rate ν I is often assumed to be equal to 10 12 s −1 .…”

Section: The Interaction Between Hydrogen/deuterium Atoms and Dust Grmentioning

confidence: 99%

Incorporation of stochastic chemistry on dust grains in the Meudon PDR code using moment equations

Petit¹,

Barzel

Biham

et al. 2009

A&A

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Context. Unlike gas-phase reactions, chemical reactions taking place on interstellar dust grain surfaces cannot always be modeled by rate equations. Because of the small grain sizes and low flux, these reactions may exhibit large fluctuations and thus require stochastic methods such as the moment equations. Aims. We evaluate the formation rates of H 2 , HD, and D 2 molecules on dust grain surfaces and their abundances in the gas phase under interstellar conditions. Methods. We incorporate the moment equations into the Meudon PDR code and compare the results with those obtained from the rate equations. Results. We find that within the experimental constraints on the energy barriers for both diffusion and desorption and the density of adsorption sites on the grain surface, H 2 , HD and D 2 molecules can be formed efficiently on dust grains. Conclusions. In a wide range of conditions, the moment equation results agree with those obtained from the rate equations. However, for a range of relatively high grain temperatures, there are significant deviations: the rate equations fail, while the moment equations provide accurate results. The incorporation of the moment equations into the PDR code can be extended to other reactions taking place on grain surfaces.

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“…In recent years, we have conducted a series of experiments on the formation of molecular hydrogen on dust grain analogs such as polycrystalline silicates (Pirronello et al 1997a(Pirronello et al , 1997b, amorphous carbon , and amorphous water ice (Manico et al 2001;Roser et al 2002Roser et al , 2003Perets et al 2005), under astrophysically relevant conditions. In these experiments, the surface was irradiated by beams of H and D atoms.…”

Section: Introductionmentioning

confidence: 99%

Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions

Perets

Lederhendler

Biham

et al. 2007

ApJ

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Experimental results on the formation of molecular hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers, we evaluate the efficiency of molecular hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H 2 formation within a temperature range that is relevant to diffuse interstellar clouds. The results also indicate that the hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.

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Molecular Hydrogen Formation on Ice Under Interstellar Conditions

Cited by 96 publications

References 38 publications

Multilayer modeling of porous grain surface chemistry

Multilayer modeling of porous grain surface chemistry

Incorporation of stochastic chemistry on dust grains in the Meudon PDR code using moment equations

Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions

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