Abundances of the various molecules observed in this study as a function of the H₂ density (upper panel). The solid lines show the abundances of the main species and the dashed lines the abundances of the deuterated isotopologues. The abundances are obtained by fitting the step functions used in the RT modelling. The central and bottom panels give the initial points used in the fit, as well as the error bars for NH₃ and N₂H⁺.