Density versus temperature contour plot for the source CSO33. The source CSO33 represents an extreme case, with the largest temperature of the analysed sample. This is probably because CSO33 is the closest to the Trapezium OB stars cluster. The minimum χ² and the 1σ contour plot are obtained for a minimum value of the χ² in the column density parameter. The values of the reduced are quoted in the upper corner of the panel.