Dependence of adiabatic indices ratio, γ₀/γ_∞, and pulse parameters on the steady state temperature, T₀, of Orion Bar PDR. Calculations are shown for different values of optical depth: τ_c = 0.5 (solid line), τ_c = 0.75 (dashed line), τ_c = 1.0 (dot-dashed line). The gray thin line, γ₀/γ_∞ = 1, divides the figure into two regions. The upper region corresponds to isentropic instability, while the lower region indicates stability. Shock-wave pulses occur where γ₀/γ_∞ > 1, i.e., T₀ ≳ 480 K.