Results from our chemical kinetics model adopting a fractional water abundance of X(H₂O) = 3 × 10^-4 (Sect. 5.1). a) Spatial distribution of the model fractional abundances in the slow central core of the envelope (fossil remnant of the AGB CSE); b) evolution with time of the model fractional abundances in a representative gas slab or cell within the lobe walls (approximately in the midpoint between the outer and inner end of the wall) where the extinction is A_V ~ 1 mag; c) model fractional abundances as a function of the depth (expressed as A_V) into the lobe walls, at ~800 yr. Dotted horizontal lines in panel c)) represent the observed beam-averaged values of our estimated fractional abundances.