Population diagram for ¹³CO and best linear fit (dashed line) to the data (filled symbols). Triangles and squares represent data points with and without the opacity correction C_τ applied, respectively (Sect. 4.1). The values of T_rot and N_tot derived from the fits are indicated in the bottom left corner of the box for the optically thin (lower) and optical depth corrected (upper) approximations. The error bars of the data points include flux uncertainties due to the rms of the spectra and absolute flux calibration, up to ~25% (Sect. 2).