Looking at the photon-dominated region in NGC 2024 through FIR line emission

We present the ISO-LWS spectra (45-200 mu m) of both the molecular cloud NG C 2024 and its associated HII region. We observed the two Class 0 objects F IR3 and FIR5 and the infrared source IRS2. All the spectra appear quite sim ilar, with approximately the same strength high-J CO rotational lines (from J(np) =17 to J(up)=14), and atomic and ionic lines from oxygen, carbon and nitrogen. This uniformity suggests the bulk of the emission is from the ex tended cloud, and is not related to the local source conditions. The molecu lar emission has been modelled with a large velocity gradient (LVG) code, a nd the results imply that the emission originates in a clumpy, extended PDR with a temperature T similar to 100K and a density n(H2) similar to 10(6) cm(-3). CO column densities in excess of 10(18) cm(-2) are derived for this : molecular component. A line intensity ratio I([O I] 63 mu m)/I([O I] 145 mu m) of about 5 is found through all the region, indicating either that th ese two lines are both optically thick at the same temperature of CO, or, m ore likely, that the 63 mu m line is strongly absorbed by cold foreground g as. The ionised emission lines have been consistently modelled with CLOUDY; the lines arise from gas illuminated by an O9.5 star or its UV equivalent, rep resenting the ionising capability of the whole OB cluster present in the re gion. From the intensity ratios of the ionic lines, relevant physical prope rties of the ionised gas (N/O abundance, electron density) are derived.