Non-thermal velocity dispersion (σ_NT) vs. gas kinetic temperature for para-H₂CO (black squares) and NH₃ (red points). For the black squares, the gas kinetic temperatures were derived from para-H₂CO 3₂₁–2₂₀/3₀₃–2₀₂ line ratios. For the red points, they were obtained from the NH₃ (2, 2)/(1, 1) line ratios. The black line is the corresponding thermal sound speed. Two sources (G5.97-1.36 and G17.10+1.02) with particularly large T_kin(NH₃) errors are not shown here.