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H2 formation on grain surface, two recent publications

Reported by: Gary J. Ferland Owned by: nobody
Priority: good to do Milestone:
Component: atomic/molecular data base Version: trunk
Keywords: Cc:


A&A 541, A76 (2012) Surface chemistry in the interstellar medium

  1. H2 formation by Langmuir-Hinshelwood and Eley-Rideal mechanisms
  2. Le Bourlot, F. Le Petit, C. Pinto, E. Roueff, and F. Roy

LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France e-mail: jacques.lebourlot@… Received 20 September 2011 / Accepted 30 January 2012 ABSTRACT Context. It has been found from ISO, Spitzer, and Herschel observations that molecular hydrogen, H2, can form on warm grains. Numerical models of interstellar chemistry have failed to reproduce the observed formation rates of H2, which remains a difficulty when interpreting observations of photon-dominated regions (PDRs). Aims. We attempt to include as much experimental and theoretical information as possible to describe H2 formation in astrophysical environments to solve this problem. Methods. We modified our “Meudon PDR code” to include a detailed treatment of H2 formation mechanisms including: i) the Langmuir-Hinshelwood mechanism taking into account the contribution of the different sizes of dust grains in the diffusion processes; and ii) the Eley-Rideal mechanism. Results. We are able to form H2 even in regions where the dust temperature is higher than 25K. We also show that formation by the Eley-Rideal mechanism can be a significant source of gas heating. We derive line intensities for various astrophysical conditions. Conclusions. Our approach results in a higher H2 formation rate than for the “standard” 3 × 10?17 nH n(H) cm3 s?1 expression.

The Astrophysical Journal, 751:58 (13pp), 2012 May 20 KINETIC MONTE CARLO STUDIES OF H2 FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE Wasim Iqbal1, Kinsuk Acharyya1,3, and Eric Herbst2 1 S. N. Bose National Centre for Basic Sciences, Salt Lake, Kolkata 700 098, India 2 Departments of Chemistry, Astronomy, and Physics, University of Virginia, Charlottesville, VA 22904, USA Received 2011 September 9; accepted 2012 March 19; published 2012 May 4 ABSTRACT We have used the continuous-time random-walk Monte Carlo technique to study the formation of H2 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 Hfluxes representative of interstellar environmentswith atomic hydrogen number density ranging between 0.1 cm?3 and 100 cm?3 and dust grain sizes ranging from 100 sites to 106 sites, the latter corresponding roughly to olivine grains of radius 0.2?m. 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 H2 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 H2 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.

Change History (1)

comment:1 Changed 3 years ago by Gary J. Ferland

Much clearer description of process

Full SED fitting with the KOSMA-$\tau$ PDR code. I. Dust modelling 
Röllig, M., Szczerba, R., Ossenkopf, V., & Glück, C.
2013, \aap, 549, A85

ABSTRACT: Aims: We revised the treatment of interstellar dust in the KOSMA-tau photo-dissociation region (PDR) model code to achieve a consistent description of the dust-related physics in the code. The detailed knowledge of the dust properties is then used to compute the dust continuum emission together with the line emission of chemical species.  Methods: We coupled the KOSMA-tau PDR code with the multi component dust radiative transfer (MCDRT) code to to solve the frequency-dependent radiative transfer equations and the thermal balance equation in a dusty clump under the assumption of spherical symmetry, assuming thermal equilibrium in calculating the dust temperatures, neglecting non-equilibrium effects. We updated the calculation of the photoelectric heating and extended the parametrization range for the photoelectric heating toward high densities and UV fields. We revised the computation of the H2 formation on grain surfaces to include the Eley-Rideal effect, thus allowing for high-temperature H2 formation.  Results: We demonstrate how the different optical properties, temperatures, and heating and cooling capabilities of the grains influence the physical and chemical structure of a model cloud. The most influential modification is the treatment of H2 formation on grain surfaces that allows for chemisorption. This increases the total H2 formation significantly and the connected H2 formation heating provides a profound heating contribution in the outer layers of the model clumps. The contribution of polycyclic aromatic hydrocarbons (PAH) surfaces to the photoelectric heating and H2 formation provides a boost to the temperature of outer cloud layers, which is clearly traced by high-J CO lines. Increasing the fraction of small grains in the dust size distribution results in hotter gas in the outer cloud layers caused by more efficient heating and cooler cloud centers, which is in turn caused by the more efficient FUV extinction.

KEYWORDS: astrochemistry, radiative transfer, methods: numerical, photon-dominated region (PDR), ISM: molecules, infrared: ISM

LOCAL FILE : ~/Dropbox/literature/molecules/H2/formation_grain/hot_grain_formation/Rollig.M13Full-SED-fitting-with-the-KOSMA-tau.pdf

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