Polar-like chart summarising the results of our parameter study. Each slice represents a model (labelled with letters) with fixed initial conditions in terms of T, ζ₂, initial density n_core, and initial H₂ OPR (see Table B.1). Within the model slice, the different collapse speed cases defined by the parameter β_ff are represented by a stacked bar. The radial evolution of the bar reflects the oH₂ D⁺/pD₂H⁺ evolution and the bars are coloured by density (see associated colour bar). The orange ring was delimited by the observed oH₂ D⁺/pD₂H⁺ ratio and the blue region in the colour bar was set by the density of the observed cores (see also Table 1). This means that we have a match between the models and the observations when the blue frame of the bar falls into the orange region.