Best fit of molecular abundances in MM1–MM3, with the chemical ages of 11 200 yr, 11 210 yr, and 11 660 yr, respectively. The last panel shows the best fit of H₂ column density, whose observed values are derived from dust emission. The modelled values and tolerance (0.1–10) are shown as a red filled region, while the observed values are plotted as solid black lines, with the uncertainties determined from T_rot, partition function Q(T_rot), and Gaussian fit to or scatter of transitions (CH₃CN and HNCO). Abundances of species (name in black at the upper-right corner of each panel) are derived from their detected rare isotopologues, or opacity correction, or from optically thin assumption, which are all named in blue in parentheses at the lower-right corner of each panel.