LVG modelling of CH₃CN. Upper panel: χ² contour plot as a function of density ( on y-axis) and temperature (x-axis) at the best fit position with respect to the CH₃CN column density and an emitting size (0″.3 in diameter). Lower panel: observed to theoretical intensities as a function of the upper level energy of the line, at the best fit position, i.e. with a CH₃CN column density N(CH₃CN) = 2 × 10¹⁶ cm⁻², an emitting size 0″.3 in diameter, kinetic temperature T_kin = 140 K, and volume density higher than 10⁷ cm⁻³.