Sticking coefficient of hydrogen and deuterium on silicates under interstellar conditions

¹ LERMA, UMR 8112 du CNRS, de l’Observatoire de Paris et de l’Université de Cergy Pontoise, 5 mail Gay Lussac, 95031 Cergy Pontoise Cedex, France
e-mail: Henda.Chaabouni@obspm.fr
² Université Paris-Sud, ISMO, UMR 8214 du CNRS, Bât. 351, 91405 Orsay, France
e-mail: herve.bergeron@u-psud.fr

Received: 5 June 2011
Accepted: 3 December 2011

Abstract

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 H₂ and D₂ 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 H₂ and D₂ molecules on a silicate surface held at 10 K. It consists of measuring with a QMS (quadrupole mass spectrometer) the signals of H₂ and D₂ 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 H₂ and D₂ 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, H₂, and D₂ 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) = S₀(1 + βT/T₀)/(1 + T/T₀)^β, which describes both the experiments and the thermal distribution expected in an astrophysical context. The parameters S₀ and T₀ 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.