Observed constraints on the CO spectral line energy distribution in PJ183+05. Our measurements are shown as black filled circles. Top and bottom panels show the models for PDRs and XDRs, respectively. We mark models, taken from Pensabene et al. (2021), with increasingly darker colors at increasing intensity of the radiation field. Dotted, short-dashed, and long-dashed lines refer to models with gas density log n [cm⁻³] = 2, 4, 6 respectively. All models are normalized to the observed CO(7–6) line flux. We plot the observed line fluxes or upper limits as black circles. In addition, we show the predicted CO(1–0) flux, as derived from the dust continuum, as described in Sect. 4.5. Our constraints exclude a strong XDR environment, but leave room for a combination of PDR and XDR environments in the excitation conditions of the molecular ISM in PJ183+05.