Top panel: fractional abundance of selected species as a function of depletion factor (see text for details). Note the sharp drop of N₂ and the shallower drop of NH₃ compared to N₂H⁺. Bottom panel: NH₃/N₂H⁺ abundance ratio as a funciton of depletion factor. Horizontal lines are the observed values (see Table 3) for protostellar (continuous) and starless (dashed) cores. Note that although all protostellar cores are consistent with both low and large values of freeze-out, two of the starless cores can only be reproduce by values of depletion factor around 1000, similar to what measured in the core nucleus of L1544 (Caselli et al. 1999).