Profiles of 𝔉 function, adiabatic temperature gradient ∇_ad, and ionization degrees of HI, HeI, and HeII in the external regions of a low-mass star. Left panel: profile of the 𝔉 function as a function of log P in the outer atmospheres of the 1.0 M_⊙ with chemical composition [X = 0.703, Y = 0.280, Z = 0.017] calculated with the MLT from the main sequence to late stages along the RGB. The adopted mixing length parameter is Λ = 1.68. The pressure is in dyn cm⁻². We highlight the two characteristic layers at log P = 5.4 and log P ≃ 6.0, where the 𝔉 function suddenly changes slope (see the text for more details). Right panel: run of adiabatic temperature gradient ∇_ad and ionization degree of HI, HeI, and HeII indicated by y_HI, y_HeI, and y_HeII, respectively, as a function of log P in dyn cm⁻². The drop off of ∇_ad at log P ≃ 5.4 coincides with the start of the HI ionization.