Comparison of observed abundances in the envelope (shown by dashed lines) with the results of chemical models involving warming up of grains for different combinations of hydrogen density (n_H2) and final warm-up temperatures. The model calculates abundances relative to hydrogen, and the observed column densities of the different species have been converted to fractional abundances assuming N(H₂) = 3.5 × 10²³ cm^-2. In all models, the warm-up occurs between 0.1 and 0.3 Myr starting from an initial temperature of 10 K, similar to the temperature profiles that appear in Hassel et al. (2008, Figs. 2 and 3).