Liseau, R.; White, G. J. and Larsson, B.
The behaviour of the [OI]63 μm and [OI] 145 μm lines in the ρOphiuchi cloud.
In: The Universe as Seen by ISO, 20-23 October 1998, Paris.
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The dense cores B1, B2 and C North in the active star forming ρOph cloud have been observed with the ISO LWS (λ = 43-197 μm, R=140-330). Detailed modelling of the FIR spectra shows that the emission originates in a PDR on the rear side of the cloud. However, the behaviour of the OI ground state lines ([OI]63 μm, [OI] 145 μm) appears anomalous in the sense that observed flux ratios are too small compared to those predicted by theoretical models of OI excitation and line transfer. Based on the LWS data, we argue that self absorption in the [OI]63 μm line by cool foreground material is not a viable explanation in the case of ρ Ophiuchi: the expected reduction in [OI]63 μm flux towards the dense cores is not reflected by the data. In contrast, the [CII] 158 μm and [OI]63 μm lines behave in accordance with model predictions. It seems likely, therefore, that it is not the [OI]63 μm line being too weak, but that the [OI] 145 μm line is relatively too strong and we propose that slight masing in the [OI] 145 μm line is responsible for the observed line ratio anomaly.
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