Theoretical studies of interstellar molecular shocks - X. The chemical fractionation of deuterium in dark clouds

The chemical fractionation of deuterium into neutral oxygen-bearing species such as OH and H₂O and ionic species such as $$\text{H}_{3}^{+}\,\text{and}\,\text{HCO}^{+}$$ is studied. The effects of a shock wave on the equilibrium state of the gas are considered. High degrees of fractionation are found in the cold, ambient medium, in agreement with previous theoretical studies. In the shock, the ratios $$\text{H}_{2}\text{D}^{+}/\text{H}_{3}^{+}\,\text{and}\,\text{DCO}^{+}/\text{HCO}^{+}$$ are much reduced, whereas HDO/H₂O remains practically ‘frozen’ at the value $$(\gt{10}^{-2})$$ in the cold gas at equilibrium. The HDO/H₂ ratio is sensitive to the value of the branching ratio in the dissociative recombination of $$\text{H}_{3}\text{O}^{+}.$$ The computed values of this ratio are compared with that deduced from observations of the hot core of Orion.