H2, HD, and D2 abundances on ice-covered dust grains in dark clouds
L. E. Kristensen1⋆, L. Amiaud2,3, J.-H. Fillion4,5, F. Dulieu1 and J.-L. Lemaire1
LAMAp/LERMA, UMR8112 du CNRS, de l’Observatoire de Paris et de l’Université
de Cergy Pontoise,
5 mail Gay-Lussac,
Cergy Pontoise Cedex,
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
Accepted: 21 December 2010
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.
Key words: ISM: molecules / molecular processes / astrochemistry
© ESO, 2011