Cooling and heating rate versus depth for the main mechanisms in a typical 1D model (stellar age: 6 Myr, L_* = 10⁹ L_⊙, n = 2, U = −2, Z = 0.1 Z_⊙, L_X = 10⁷ L_⊙, T_X = 10⁶ K). The dashed vertical lines correspond to the cut values 1, 2, and 3. Acronyms: “IScclinH”: hydrogen recombination; “ISrcolH”: net free-bound cooling minus free-free heating; “PE”: photoelectric effect heating; “CR”: cosmic ray heating.