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 IIregion-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 (mu)m, [ N III] 57 (mu)m, [ O I] 63 (mu)m, [ O III] 88 (mu) m, [ N II] 122 (mu)m, [ O I] 146 (mu)m, [ C II] 158 (mu)m, and [ Si II] 35 (mu)m together with the far-infrared continuum and the H 2 pure rotation transition ( J=5-3) line at 9.66 (mu)m. 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 (mu)m emission was observed and the [ O I] 63 (mu)m to 146 (mu)m 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 [ O I] 63 (mu)m and the [ C II] 158 (mu)m emission by overlapping PDRs along the line of sight can account for the observations and that the [ O I] 146 (mu)m 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 (mu)m emission is quite strong at almost all the observed positions. The correlation with [ N II] 122 (mu)m suggests that the [ Si II] emission originates mostly from the ionized gas. The [ Si II] 35 (mu)m to [ N] 122 (mu)m 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.