PEP results for ¹³CO spectra towards a BE sphere with T_kin = 10 K and M = 2 M_⊙. The colour scale shows the predicted line intensities as a function of volume and column density. In the green hatched areas PEP predictions are within 10% of the observed line intensities. The circles indicate the reference cloud parameters for the central LOS (small circle), along with the beam average with FWHM = R₀/3 (middle circle corresponding to the beam in the synthetic observations) and for FWHM = R₀ (large circle).