Giannini, T.; Nisini, B.; Lorenzotti, D.; Di Giorgio, A. M.; Spinoglio, L.; Benedettini, M.; Saraceno, P.; Smith, H. A. and White, G. J.
Looking at the photon-dominated region in NGC 2024 through FIR line emission.
Astronomy and Astrophysics, 358
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We present the ISO-LWS spectra (45-200 μm) 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 Jup=17 to Jup=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 nH2 ~ 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 μm)/I([O I] 145 μm) 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 μm 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.
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