PDR simulation of an n _H = 300 cm⁻³ total number density interacting with various FUV intensities in the range G ₀ = 1 − 10⁶. Top left: emissivity of [C II] vs. the visual extinction, A _V . As G ₀ increases, [C II] emissivity increases from ∼6.5 × 10⁻²⁴ erg s⁻¹ cm⁻³ to ∼1.7 × 10⁻²³ erg s⁻¹ cm⁻³, at which point it saturates, approaching asymptotically a maximum value. Bottom left: emissivity of FIR calculated assuming that the total dust cooling is equal to radiative dust heating (Equation (8)), vs. A _V . Under the optically thin assumption, the FIR emission is given by integrating Equation (8) along the line of sight. Top right: gas temperature vs. A _V when thermal balance has been reached. The temperature at the surface of the PDR increases from ∼120 K to ∼1.6 × 10³ K as G ₀ increases. Bottom right: [C II]/FIR vs. Σ_FIR assuming optically thin emission. Due to the thermal saturation of the [C II] emissivity, the ratio [C II]/FIR decreases for G ₀ > 10 in these simulations, leading to a [C II] deficit medium. The gray triangles represent the simulation data as discussed in Figure 8.