Variation of the para–D₂H⁺ to ortho–H₂D⁺ ratio as a function of the gas temperature, for densities of 1 × 10⁵ (green), 5 × 10⁵ (blue) and 1 × 10⁶ (red) cm^-3. The thick lines show the ratio derived from observations, using non-LTE modeling, whereas the thin lines show the chemical model results. Three depletion factors at steady-state are presented: complete depletion (CD: solid lines), depletion factor =100 (dot-dashed lines) and depletion factor =10 (dashed lines). The thin dashed vertical lines present the [9.5–12.4] K range.