Selected input and output quantities of the PDR models shown as a function of the distance from the white dwarf. We consider three models, shown from left to right, in which the density is one-third, half, and one times that of the extended heuristic model (see text), respectively. The different panels show from top to bottom (a) the density of H nuclei n_H (note the peak reached at a distance of ~2 × 10¹⁴ cm for Λ = 0°, which stands for the position of the AGB star), (b) the visual extinction A_V measured from the WD, (c), the fractional abundance of CO relative to H nuclei n(CO)/N_H, and (d) the photodissociation rate of CO, Γ(CO). The verticaldotted line at the left edge is the position of the Strömgren radius.