Contours of χ², indicating the sensitivity of the fit to the model’s free parameters. The left panel shows the dependence of χ² on the photodissociation radius R_ph and abundance of ortho-NH₃, given the best fitting value for the formation radius, R_f = 2.5 R_⋆. The middle and the right panels show the dependence of χ² on the formation radius R_f and the abundances of para- and ortho-NH₃, given a photodissociation radius R_ph = 3 × 10¹⁶ cm. The contours correspond to 1, 2, and 3σ confidence levels. The best fits are marked with a cross and the corresponding are shown on the plots.