Observed vs. predicted [Fe II] 26 µm/[S I] and [Fe I]/[S I] line ratios for different knots (red triangles) along the jet. Each curve reproduces the Hollenbach & McKee (1989) dissociative models for different values of the gas-phase iron abundance (coloured curves, from 2.88 × 10⁻⁵ – black dots – to 7 × 10⁻⁷ – magenta open circles), pre-shock density n₀=10⁵ cm⁻³, and a range of shock velocities (v_s=30–100 km s⁻¹), as reported in the labels. The two blue arrows show how the plots move by varying n₀ from 10⁴ to 10⁶ cm⁻³.