Computed molecular column densities for DCO⁺ (dashed line) and HCO⁺ (solid line). Black lines show the columns obtained with the best-fit model (see Sect. 3.3), red lines show the columns when the specified physical parameter is changed in the best fit model while the others are held constant. For reference, the canonical model has a single grain population of a = 0.1 μm, L_X = 3 × 10²⁹ erg s^-1, χ_∗(100 AU) = 410 and a CRP ionization rate of ζ = 1.3 × 10^-17 s^-1.