Observations of deuterated cyanoacetylene in dark clouds

We have observed DC3N and HC3N in a number of cold dust clouds in order to derive the degree of deuterium fractionation. We find that the ratio of DC3N to HC3N is large, at about 0.05 or more, and discuss the implications of this result for the synthesis of cyanoacetylene. The observations are most readily interpreted if the deuteration of HC3N is linked to that of cyclic C3H2, which is also observed to exhibit a large degree of deuterium fractionation. HC3N deuteration levels comparable with those we observed are found to be just compatible with the mechanism suggested by Howe & Millar, but with adjusted rate coefficients. Freeze-out on to grain surfaces is also considered, but produces widespread deuterium enhancement in many species, contrary to observed levels.