Deuterium in interstellar clouds

Despite the fact that the amount of deuterium made in the Big Bang is about 10(-5) compared to hydrogen, interstellar molecules contain D/H ratios that are enhanced by a process called fractionation. Such enhancements are caused by zero-point-energy effects that operate at the very low temperatures of interstellar clouds. The details of the fractionation process have been known for about 15 years but recent observations of multiply deuterated molecules including ND3 and CD3OH, with enhancements of around 10(11) times the cosmic ratio, have forced us to re-examine the theory surrounding fractionation. It turns out that the theory remains secure but that these large enhancements are reflecting physical conditions in the late stages of cloud collapse when most gas-phase molecules are frozen out on to the surfaces of interstellar grains. As a result, observations of deuterated ions, such as H2D+ and D2H+, may be the best probes of this stage in the formation of stars.