Two-dimensional cuts through the χ² topology of the RADEX simulation of the observed intensities of the CO 1−0, 2−1 and ¹³CO 1−0 and 2−1 lines. The χ² values are given in logarithmic units, so that only the dark blue area indicates a valid fit. All slices are taken through the χ² minimum. The contour lines show the parameters at which the observed values were matched. The black and white lines show the integrated intensities of CO 2−1 and ¹³CO 2−1, and the red and brown lines show the ratios of CO 1−0/CO 2−1 and ¹³CO 1−0/¹³CO 2−1.