The HCN depletion as a function of the CO depletion during the collapse, computed by the gravitational collapse model of Paper I, see their Fig. 8. Values are normalised with respect to the initial abundances. Each point is labelled with the time of the collapsing gas. Two models are shown: a free fall model labelled t_ff (red dashed curve) and a collapsing model with a time scale multiplied by a factor ten, labelled 10   t_ff (blue solid curve). The black dotted curve shows the positions of equal depletion for CO and HCN.