Variations in the radial distribution of HCN (Σ₃₀₀ and p parameters) according to the maximum grain size a ₊ , the UV field intensity (χ factor), the rate of cosmic ray ionization, and modification of the physical structure. The “standard” values are χ = 10³, CR = 10^-17 s^-1, and nominal temperature and density structure. Squares: big grains (a ₊  = 0.1 mm); triangles: small grains (a ₊  = 0.3 μm).