Population diagram of observed AlF rotational transitions towards o Ceti that are listed in Table 2. Blue points correspond to flux densities that are extracted from region 1 and green points correspond to those from region 2 (see Sect. 3.1 for explanations). The vibrationally excited v = 1 lines in both regions are also shown by x symbols, but they were not considered in the calculations of the rotational temperatures. The vertical error bars are calculated from the uncertainties of the integrated fluxes (uncertainty from the Gaussian fitting plus 10% of the total flux from the calibration of the data) and are considered in the fitting process.