H₂, HD, and D₂ abundances on ice-covered dust grains in dark clouds

¹ LAMAp/LERMA, UMR8112 du CNRS, de l’Observatoire de Paris et de l’Université de Cergy Pontoise, 5 mail Gay-Lussac, 95000 Cergy Pontoise Cedex, France
e-mail: jean-louis.lemaire@obspm.fr
² Univ. Paris-Sud, ISMO, UMR 8214, 91405 Orsay, France
³ CNRS, UMR 8214, ISMO, 91405 Orsay, France
⁴ UPMC Univ. Paris 6, UMR 7092, LPMAA, 75005 Paris, France
⁵ CNRS, UMR 7092, LPMAA, 75005 Paris, France

Received: 20 March 2009
Accepted: 21 December 2010

Abstract

Aims. We seek to study the abundances of H₂, HD, and D₂ adsorbed onto ice-covered dust grains in dark molecular clouds in the interstellar medium.

Methods. We use our previously developed detailed model describing temperature-programmed desorption (TPD) experiments of H₂ and its isotopologues on water ice. We here extrapolate these model results from laboratory conditions to conditions similar to those found in dark molecular clouds.

Results. By means of our model we are able to infer three important results. (i) The time scale for H₂ and isotopologues to accrete onto dust grains is less than 10⁴ yrs. (ii) Due to the higher binding energy of D₂ with respect to HD, D₂ becomes the most abundant deuterated species on grains by  ~50% with respect to HD (a few times 10^-5 with respect to H₂). (iii) The surface coverage of D₂ as a function of temperature shows that at very low temperatures (i.e., less than 10 K), D₂ may be two orders of magnitude more abundant than HD. Possible implications for deuteration of water on grain surfaces are discussed when it forms through reactions between OH and H₂.