Top: comparison of integrated line fluxes of ¹²CO observed by Herschel/PACS and from the ground (black dots with errorbars) and the predictions of the quiescent envelope passively heated by the luminosity of the source (red crosses) for NGC 7538-IRS1. Bottom: the same model compared with the JCMT ¹²CO 3–2 and Herschel/HIFI ¹³CO 10–9 and C¹⁸O 9–8 observed line profiles. An additional model including an outflow component is shown in blue dashed line.