Modeled L₂/L_IR ratio as a function of τ₁₀₀. Squares and triangles indicate T_dust = 55 and 75 K, respectively. In all models, collisional excitation is included with T_gas = 150 K and n_H2 = 3 × 10⁴ cm^-3. N_H2O/τ₁₀₀ = 10¹⁸ cm^-2 is adopted, corresponding to a constant H₂O abundance of 7.7 × 10^-7 (Eq. (1)). Blue symbols indicate models with ΔV = 100 kms^-1 and thus with variable K_vir (Eq. (5)) indicated with the numbers. Green symbols show results with ΔV = 100 × τ₁₀₀ kms^-1 simulating a constant value of K_vir = 1.3. When compared with observations, the modeled L_IR values should be considered a fraction of the observed IR luminosities (Sect. 4.3.1), and thus the modeled L₂/L_IR values are upper limits.