Journal Article

Are molecule-covered dust grains efficient catalysts of H<sub>2</sub> formation in the cold ISM?

L. Gavilan, J. L. Lemaire and G. Vidali

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 424, issue 4, pages 2961-2970
Published in print August 2012 | ISSN: 0035-8711
Published online August 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.21463.x
Are molecule-covered dust grains efficient catalysts of H2 formation in the cold ISM?

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Abstract

The formation of H2 in the interstellar medium (ISM) involves complex processes, some of which are still not understood. In cold regions, it is assumed that H2 formation follows Langmuir kinetics, i.e. the immediate desorption of incoming atoms or molecules on a surface already saturated with molecules. Our experiments address this issue by studying the formation of H2 on a dust surface dosed with molecules prior to atomic exposure. We simulate ISM conditions at Tdust < 10 K and Tgas ∼ 90 K and use a synthesized amorphous silicate. By coupling laser detection to thermal desorption spectroscopy, we confirm that hydrogen recombination is promptly enhanced. We interpret this as a result of enhanced atomic diffusion (both hopping thermal and quantum mechanical tunnelling). Moreover, since H2 formation is the most exothermic chemical reaction per unit mass, we elucidate its importance as a non-thermal desorption mechanism. We apply these results to dense ISM regions where H2 formation and its induced desorption are curbed by a declining atomic gas-phase abundance. We further propose this as a pathway to deuterium fractionation in pre-stellar cores. More importantly, we show that dust remains an active catalyst even in the coldest ISM.

Keywords: astrochemistry; molecular processes; methods: laboratory; ISM: atoms; ISM: molecules

Journal Article.  7758 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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