Comparison of five observed spectra (black histograms) and the same ones plus the required amount of noise to saturate their peak S/N to ten (red histograms). The positions of the spectra are marked on Fig. 1 and listed in Table 2. Each row corresponds to a given species and transition, while each column corresponds to a given position. In each panel, the top left and right numbers give the relative weight of a difference of 1 K in any given channel on the fit χ² value before and after the S/N saturation, respectively. The ¹²CO (1 − 0) and H¹³CO⁺ (1 − 0) lines were observed with a resolution about 5 times coarser than the other lines. When the peak S/N is lower than 10, the red spectrum is hidden under the black one.