Sticking coefficient of hydrogen and deuterium on silicates under interstellar conditions

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 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.

Modeling Dust Evolution in Galaxies With a Multiphase, Inhomogeneous Ism

“…This is not surprisingsince the growth is limited to  T 300 K in the reference model. Our results do not drastically change for models with different temperature-dependent sticking coefficients tested in this work (Leitch-Devlin & Williams 1985;Chaabouni et al 2012). Compared to a simple step function dependence on temperature in the reference models, the sticking coefficient usually decreases with gas temperatures from the maximum value close to one,similar to the behavior of the data from Chaabouni et al (2012) Our simulations do not resolve the dense gas well, soits actual contribution can be higher than the present value of a few percent.…”

“…We deem the temperature dependence of the sticking coefficient measured by Chaabouni et al (2012) more plausible than that of the α derived by Leitch-Devlin & Williams (1985) (Figure 1). The former increases at low gas temperatures and approaches one at  T 10 gas K, while the latter peaks at » T 200 K gas and decreases at lower gas temperatures.…”

“…With these uncertainties, the observed relation between interstellar depletions and gas density may provide an observational constraint for the choice of α. To this end, we investigate various choices of the sticking coefficient: (1) a fixed value a = 1 for  T 300 gas K and a = 0 for > T 300 gas K,(2) a fixed value a = 1 for all temperatures,(3) anexperimentally measured α for chemisorption of H 2 molecules on silicate surfaces by Chaabouni et al (2012),and (4) a T T , gas dust ( ) from theoretical calculations of physisorption performed by Leitch-Devlin & Williams (1985). In theabsence of estimates of α for physisorption of Si on silicate grain surfaces, we provisionally adopt the data for carbon atoms arriving on a graphite lattice from the work by Leitch-Devlin & Williams (1985), in the functional form derived by Grassi et al (2011).…”

Grain Surface Models and Data for Astrochemistry

“…These studies typically focus on the accretion of a single atom or molecule on an otherwise bare surface, whereas the sticking coefficient could be coverage dependent, especially for chemisorption where for high coverage there are simply fewer sites available for sticking. Experimentally it was found that the sticking coefficient of physisorbed H 2 increases linearly with the number of deuterium molecules already adsorbed on the surface (Amiaud et al 2007;Chaabouni et al 2012). …”

Disk Chemistry

“…In the particular case of H sticking on silicate or amorphous ice surfaces, many experimental and theoretical studies have been performed. For example, Chaabouni et al (2012) propose a parametric formula that fits experimental and theoretical values…”