The role of H2D+ in the deuteration of interstellar molecules

Collisions between H-3(+) and HD in molecular clouds lead to the fractionat ion of deuterium in H2D+ at temperatures below 20 K. In this article, we de scribe the chemistry of H2D+ and discuss how variations in temperature and elemental abundances affect the level of fractionation in H2D+ and other sp ecies. We describe how accretion of gas-phase molecules on to cold dust gra ins enhances the deuteration in several molecules including doubly deuterat ed molecules. Mie show that the ion-neutral drift velocities attained in sl ow Alfven waves can destroy H2D+ in non-thermal reactions. As a result, the degree of fractionation can be reduced and we discuss observational conseq uences of such a model for the dark dust cloud TMC-1.