Left: fractional abundances with respect to hydrogen as a function of time during phase I for the central core (top panel) and the western/eastern cores (middle panel). The NH₃/N₂H⁺ abundance ratio for both the central core and the western/eastern core are also shown in the bottom panel. Right: same as left panels but for phase II calculation. The y-axis ranges according to the model. In the bottom right panel we also show the temperature as a function of time, indicated by the thin lines. The two shaded areas mark the range of values (400–1000 for the central core and 50–200 for the western/eastern cores) of the NH₃/N₂H⁺ abundance ratio estimated from the observational data.