Models of the gas-grain interaction - deuterium chemistry

Chemical models of a dense quiescent cloud, including accretion, and a hot molecular core have been extended to include deuterium chemistry, both in the gas phase and on the surface of dust grains. For the dense cloud model we find that as accretion begins to dominate the chemistry, and atoms and molecules leave the gas phase, the destruction rate of many radicals is reduced. As a result their abundances increase sharply before decreasing as they too accrete on to the dust. This is particularly the case for deuterated species. We suggest that if observations of a cloud, such as TMC-1, reveal a high abundance of OD, for example, this would indicate that accretion was dominating the chemistry in that region. In the model of a hot molecular core we are able to explain the enhanced abundances of NH2D and HDO, and the high deuterium fractionation observed by including grain surfaces reactions involving deuterium atoms. (author)