Mid- to far-infrared spectroscopy of Sharpless 171

Abstract

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 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 H₂ 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 Ii] 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.

Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the UK) and with the participation of ISAS and NASA.