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

Lars E. Kristensen (1, 2) , Lionel Amiaud (3) , Jean-Hugues Fillion (4) , François Dulieu (1, 2) , Jean-Louis Lemaire (1, 2)

1 LERMA - Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique
2 Molécules dans l'Univers
3 ISMO - Institut des Sciences Moléculaires d'Orsay
4 LPMAA - Laboratoire de Physique Moleculaire pour l'Atmosphere et l'Astrophysique

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.

Domaines: Astrophysique [astro-ph]

Lars E. Kristensen, Lionel Amiaud, Jean-Hugues Fillion, François Dulieu, Jean-Louis Lemaire. H2, HD, and D2 abundances on ice-covered dust grains in dark clouds. Astronomy and Astrophysics - A&A, 2011, 527, pp.44. ⟨10.1051/0004-6361/200912124⟩. ⟨hal-03742141⟩