Photoelectric heating efficiency ϵ, equated as cooling efficiency [C II]/FIR, in M42 (top left: Veil Shell, bottom left: OMC4 with OMC1 (in gray scale)), the shell of M43 (top right), and the shell of NGC 1977 (bottom right) as function of the ionization parameter γ = G₀ T^1∕2∕n_e. Colors indicate the distance from θ¹ Ori C in the Orion Nebula, from NU Ori in M43, and from 42 Orionis in NGC 1977, respectively. Contours in the upper-left and lower-right panels indicate the density levels above which 90, 70, 50, 30, and 10% of the points lie. The red curve is the theoretical prediction of Eq. (43) by Bakes & Tielens (1994).