Modelled D/H abundance ratio for CH as a function of temperature computed for times t = 5 × 10⁵ yr (solid curve with diamond markers) and 1 × 10⁶ yr (dashed curve) for models with χ_UV = 1 (left) and χ_UV = 6 (right), where the horizontal grey shaded regions mark the dispersion in the CD/CH abundance ratio derived toward IRAS 16293-2422. For comparison, we also display the temperature dependence on the D/H abundance ratio for HCO⁺ (blue triangles) and C₂H (red circles) at the time of t = 5 × 10⁵ yr.