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

Abstract

We present the ISO-LWS spectra (45-200 mu m) of both the molecular cloud NGC 2024 and its associated HII region. We observed the two Class 0 objects FIR3 and FIR5 and the infrared source IRS2. All the spectra appear quite similar, with approximately the same strength high-J CO rotational lines (from J_up=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 extended cloud, and is not related to the local source conditions. The molecular emission has been modelled with a large velocity gradient (LVG) code, and the results imply that the emission originates in a clumpy, extended PDR with a temperature T ~ 100 K and a density n_{H_2} ~ 10⁶ cm^-3. CO column densities in excess of 10¹⁸ 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 these two lines are both optically thick at the same temperature of CO, or, more likely, that the 63 mu m line is strongly absorbed by cold foreground gas. 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, representing the ionising capability of the whole OB cluster present in the region. From the intensity ratios of the ionic lines, relevant physical properties of the ionised gas (N/O abundance, electron density) are derived. Based on observations with ISO, an ESA project with instruments funded by ESA Member States and with the participation of ISAS and NASA