2003A&A...412..199O

We have collected one-dimensional raster-scan observations of the active star-forming region Sharpless 171 (S171), a typical HII region-molecular cloud complex, with the three spectrometers (LWS, SWS, and PHT-S) on board ISO. We have detected 8 far-infrared fine-structure lines, [OIII] 52µm, [NIII] 57µm, [OI] 63µm, [OIII] 88µm, [NII] 122µm, [OI] 146µm, [CII] 158µm, and [SiII] 35µm together with the far-infrared continuum and the H₂ pure rotation transition (J=5-3) line at 9.66µm. The physical properties of each of the three phases detected, highly-ionized, lowly-ionized and neutral, are investigated through the far-infrared line and continuum emission. Toward the molecular region, strong [OI] 146µm emission was observed and the [OI] 63µm to 146µm line ratio was found to be too small (∼5) compared to the values predicted by current photodissociation region (PDR) models. We examine possible mechanisms to account for the small line ratio and conclude that the absorption of the [OI] 63µm and the [CII] 158µm emission by overlapping PDRs along the line of sight can account for the observations and that the [OI] 146µm emission is the best diagnostic line for PDRs. We propose a method to estimate the effect of overlapping clouds using the far-infrared continuum intensity and derive the physical properties of the PDR. The [SiII] 35µm emission is quite strong at almost all the observed positions. The correlation with [NII] 122µm suggests that the [SiII] emission originates mostly from the ionized gas. The [SiII] 35µm to [NII] 122µm ratio indicates that silicon of 30% of the solar abundance must be in the diffuse ionized gas, suggesting that efficient dust destruction is undergoing in the ionized region.