Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions

Perets, Hagai B.; Lederhendler, Adina; Biham, Ofer; Vidali, Gianfranco; Li, Ling; Swords, Sol; Congiu, E.; Roser, J. E.; Manicó, Giulio; Brucato, J. R.; Pirronello, V.

doi:10.1086/518862

Cited by 65 publications

(108 citation statements)

References 25 publications

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“…Several laboratories have worked on H atom irradiation and adsorption on diverse surfaces such as graphite and other carbonaceous material, silicates, and ice mantles (mainly water ice), followed by chemistry leading to H 2 production (e.g. Gavilan et al 2012;Vidali et al 2009;Mennella 2008;Amiaud et al 2007;Perets 2007;Creighan et al 2006, and references therein). Many of these studies focused principally on physisorbed atoms and are in general poorly efficient in producing large H 2 abundances at temperatures above about 30 K. The observation of H 2 formation in astronomical environments with gas at high kinetic temperatures has led to the proposition of a mechanism that necessarily involves chemisorbed H atoms (Habart et al 2005;Cazaux et al 2011;Le Bourlot et al 2012) to overcome the barrier of higher temperature formation, in particular in PDR environments.…”

Section: Astrophysical Implicationsmentioning

confidence: 99%

Vacuum ultraviolet photolysis of hydrogenated amorphous carbons

Alata

Cruz-Díaz

Muñoz

et al. 2014

A&A

119

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Context. The interstellar hydrogenated amorphous carbons (HAC or a-C:H) observed in the diffuse medium are expected to disappear in a few million years, according to the destruction time scale from laboratory measurements. The existence of a-C:H results from the equilibrium between photodesorption, radiolysis, hydrogenation and resilience of the carbonaceous network. During this processing, many species are therefore injected into the gas phase, in particular H 2 , but also small organic molecules, radicals or fragments. Aims. We perform experiments on interstellar a-C:H analogs to quantify the release of these species in the interstellar medium. Methods. The vacuum ultraviolet (VUV) photolysis of interstellar hydrogenated amorphous carbon analogs was performed at low (10 K) to ambient temperature, coupled to mass-spectrometry detection and temperature-programed desorption. Using deuterium isotopic substitution, the species produced were unambiguously separated from background contributions. Results. The VUV photolysis of hydrogenated amorphous carbons leads to the efficient production of H 2 molecules, but also to small hydrocarbons. Conclusions. These species are formed predominantly in the bulk of the a-C:H analog carbonaceous network, in addition to the surface formation. Compared with species made by the recombination of H atoms and physisorbed on surfaces, they diffuse out at higher temperatures. In addition to the efficient production rate, it provides a significant formation route in environments where the short residence time scale for H atoms inhibits H 2 formation on the surface, such as PDRs. The photolytic bulk production of H 2 with carbonaceous hydrogenated amorphous carbon dust grains can provide a very large portion of the contribution to the H 2 molecule formation. These dust grains also release small hydrocarbons (such as CH 4 ) into the diffuse interstellar medium, which contribute to the formation of small carbonaceous radicals after being dissociated by the UV photons in the considered environment. This extends the interstellar media environments where H 2 and small hydrocarbons can be produced.

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Section: Astrophysical Implicationsmentioning

confidence: 99%

Vacuum ultraviolet photolysis of hydrogenated amorphous carbons

Alata

Cruz-Díaz

Muñoz

et al. 2014

A&A

119

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“…Experimental studies for the interaction between H or D with silicate surfaces exist in the literature (Pirronello et al 1997;Perets et al 2007;Vidali et al 2007Vidali et al , 2009), but no experimental estimates for the sticking are indicated. In addition, the assumption of no isotopic effects is usually made.…”

Section: The Experimental Situationmentioning

confidence: 99%

Sticking coefficient of hydrogen and deuterium on silicates under interstellar conditions

Chaabouni

Bergeron

Baouche

et al. 2012

A&A

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Context.Sticking of H and D atoms on interstellar dust grains is the first step in molecular hydrogen formation, which is a key reaction in the interstellar medium. Isotopic properties of the sticking can have an incidence on the observed HD molecule. Aims. After studying the sticking coefficients of H 2 and D 2 molecules on amorphous silicate surfaces experimentally and theoretically, we extrapolate the results to the sticking coefficient of atoms and propose a formulae that gives the sticking coefficients of H and D on both silicates and icy dust grains. Methods. In our experiments, we used the King and Wells method for measuring the sticking coefficients of H 2 and D 2 molecules on a silicate surface held at 10 K. It consists of measuring with a QMS (quadrupole mass spectrometer) the signals of H 2 and D 2 molecules reflected by the surface during the exposure of the sample to the molecular beam at a temperature ranging from 20 K to 340 K. We tested the efficiency of a physical model, developed previously for sticking on water-ice surfaces. We applied this model to our experimental results for the sticking coefficients of H 2 and D 2 molecules on a silicate surface and estimated the sticking coefficient of atoms by a single measurement of atomic recombination and propose an extrapolation. Results. Sticking of H, D, HD, H 2 , and D 2 on silicates grains behaves the same as on icy dust grains. The sticking decreases with the gas temperature, and is dependent on the mass of the impactor. The sticking coefficient for both surfaces and impactors can be modeled by an analytical formulae S (T ) = S 0 (1 + βT/T 0 )/(1 + T/T 0 ) β , which describes both the experiments and the thermal distribution expected in an astrophysical context. The parameters S 0 and T 0 are summarized in a table. Conclusions. Previous estimates for the sticking coefficient of H atoms are close to the new estimation; however, we find that, when isotopic effects are taken into account, the sticking coefficient variations can be as much as a factor of 2 at T = 100 K.

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“…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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Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions

Cited by 65 publications

References 25 publications

Vacuum ultraviolet photolysis of hydrogenated amorphous carbons

Vacuum ultraviolet photolysis of hydrogenated amorphous carbons

Sticking coefficient of hydrogen and deuterium on silicates under interstellar conditions

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

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