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.