Deuterium fractionation in the disk. a) Comparisons of HCO⁺, blue, and DCO⁺, red, from observations, dashed with 3σ errors, and the best fit model, solid with associated error of a factor of 3. They grey vertical bars show where the observationally derived column densities are extrapolated beyond the inner and outer edges found in parameterisation. b) and c) show the relative abundances of DCO⁺ and HCO⁺ in the best fit disk chemical model respectively. d) shows the local DCO⁺/HCO⁺ ratio in the disk model.