Example of optically thick CO emission from a 10 M_⊙ BE model with T_kin = 10 K. Frame a shows spectra, frame b T_ex profiles, and frame c the radial variation in the photon escape probability β. Results are shown for full non-LTE calculations with LOC (solid lines), for constant excitation equal to the mean values of the LOC solution (“⟨LOC⟩”, dashed lines), and for the LTE case at T_kin (dotted lines). Frames a–b show data for the first three transitions (black, blue, and red, respectively) but, for clarity, frame c includes only the J = 1–0 and J = 3–2 transitions.