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 µm, [N iii] 57 µm, [O i] 63 µm, [O iii] 88 µm, [N ii] 122 µm, [O i] 146 µm, [C ii] 158 µm, and [Si ii] 35 µm together with the far-infrared continuum and the H2 pure rotation transition (J = 5–3) line at 9.66 µm. 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 µm emission was observed and the [O i] 63 µm to 146 µm 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 µm and the [C ii] 158 µm emission by overlapping PDRs along the line of sight can account for the observations and that the [O i] 146 µm 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 µm emission is quite strong at almost all the observed positions. The correlation with [N ii] 122 µm suggests that the [Si ii] emission originates mostly from the ionized gas. The [Si ii] 35 µm to [N ii] 122 µm 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.