Mean excitation temperature T_ex(F_u–F_l) of the three HCN J = 1–0 HFS lines obtained from nonlocal non-LTE models of constant density and temperature clouds with Δv_turb = 1 km s⁻¹. N(HCN) is 10¹⁴ cm⁻² in the upper panels and 10¹³ cm⁻² in the lower panels. Dotted curves show models that neglect line overlaps between different HFS lines. The gray-shaded areas are regions of the parameter space where the excitation is very suprathermal (T_ex ≫T_k) or weakly masing (T_ex < 0 with small | τ |).