Deuterium fractionation along the TMC-1 ridge

In this paper, we present a model to predict and explain the observed gradi ents in deuterium fractionation and molecular abundance along the TMC-1 rid ge. In TMC-1, gradients in the level of deuterium fractionation are observe d for all the molecular species for which the measurement has been made; HC O+, HC3N, C3H2 and HNC. The model is based on the assumption that the chemi cal evolution of the TMC-1 ridge has been affected by Alfven waves. The str ucture of the model and its comparison with observations are discussed. We find that the model qualitatively and somewhat quantitatively reproduces th e observations of the aforementioned molecular species. We present predicti ons of the fractionation gradients along the TMC-1 ridge for other molecule s of observational interest.