From the leftmost to the rightmost column: ratio of best model prediction over observed integrated intensities (in log₁₀ scale, the colour scale limits are a factor of two in red and a factor 0.5 in blue), best model predictions, observations, S/N level, histograms of the residuals normalised by the observations and finally best model prediction as a function of observations. For the observation column, red contours in each panel indicate ¹³CO(J = 1 → 0) integrated intensities of 4 and 12 K km s⁻¹ (dashed and solid contours, respectively). Each row is a different emission line, from top to bottom: ¹²CO(J = 1 → 0), ¹³CO(J = 1 → 0), C¹⁸O(J = 1 → 0), HCO⁺(J = 1 → 0), HNC(J = 1 → 0), HCN(J = 1 → 0) and N₂H⁺(J = 1 → 0).