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  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  )5 2µm, (N  )5 7µm, (O  )6 3µm, (O  )8 8µm, (N ) 122 µm, (O ) 146 µm, (C ) 158 µm, and (Si  )3 5µm 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 investi- gated through the far-infrared line and continuum emission. Toward the molecular region, strong (O ) 146 µm emission was observed and the (O  )6 3µm 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  )6 3µ ma nd the (C) 158 µm emission by overlapping PDRs along the line of sight can account for the observations and that the (O ) 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  )3 5µm emission is quite strong at almost all the observed positions. The correlation with (N ) 122 µm suggests that the (Si ) emission originates mostly from the ionized gas. The (Si  )3 5µ mt o (N) 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.