Modelled fractional abundances of HCO⁺ (in purple) and DCO⁺ (in yellow) with respect to the total gas density (n_H) for models with ζ_p(H) and χ_UV = (1.3 × 10⁻¹⁶ s⁻¹ 6) (left) and (1.3 × 10⁻¹⁷ s⁻¹, 6) (right), respectively. The solid and dashed curves represent the fractional abundances for A_v = 3 and 4 mag, respectively, while the purple and yellow shaded regions represent the range of HCO⁺ and DCO⁺ abundances, respectively, as determined from values reported in van Dishoeck et al. (1995). The vertical grey shaded region represents the approximate age of the source.