Mid- to far-infrared spectroscopy of Sharpless 171

We have collected one-dimensional raster-scan observations of the active star-forming region Sharpless 171 (S171), a typical H II 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, [O III] 52 mum, [N III] 57 mum, [O I] 63 mum, [O III] 88 mum, [N II] 122 mum, [O I] 146 mum, [C II] 158 mum, and [Si II] 35 mum together with the far-infrared continuum and the H-2 pure rotation transition (J=5-3) line at 9.66 mum. 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 [O I] 146 mum emission was observed and the [O I] 63 mum to 146 mum line ratio was found to be too small (similar to5) 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 [O I] 63 mum and the [C II] 158 mum emission by overlapping PDRs along the line of sight can account for the observations and that the [O I] 146 mum 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 [Si II] 35 mum emission is quite strong at almost all the observed positions. The correlation with [N II] 122 mum suggests that the [Si II] emission originates mostly from the ionized gas. The [Si II] 35 mum to [N II] 122 mum 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.