Evolution of the N₂D⁺/N₂H⁺ ratio with temperature for the slow (right) and fast (left) models. In the reference model, the temperature of the chemical network is the temperature of the gas. The two other cases are for constant temperature imposed on the chemical network. To see the differences better, the reference model has been drawn with the full line mode (from respectively the slow and fast cases displayed in Fig. 5).