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] 52μm, [N III] 57 μm, [O I] 63 μm, [D III] 88μm, [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.