Deuterium fractionation along the TMC-1 ridge

In this paper, we present a model to predict and explain the observed gradients in deuterium fractionation andmolecular abundance along the TMC-1 ridge. In TMC-1, gradients in the level of deuterium fractionation are observed for all the molecular species for which the measurement has been made; ${\rm HCO^+}$, ${\rm HC_3N}$, ${\rm C_3H_2}$and HNC. The model is based on the assumption that the chemical evolution of the TMC-1 ridge has been affected by Alfvén waves. The structure of the model and its comparison with observations are discussed. We find that the model qualitatively and somewhat quantitatively reproduces the observations of the aforementioned molecular species. We present predictions of the fractionation gradients along the TMC-1 ridge for other molecules of observational interest.