Volume 412, Number 1, December II 2003
|Page(s)||199 - 212|
|Section||Interstellar and circumstellar matter|
|Published online||25 November 2003|
Mid- to far-infrared spectroscopy of Sharpless 171*
Department of Astronomy, School of Science, University of Tokyo, Tokyo 113-0033, Japan
2 Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan
3 Department of Earth Science and Astronomy, University of Tokyo, Tokyo 153-8902, Japan
Corresponding author: Y. Okada, firstname.lastname@example.org
Accepted: 11 September 2003
We have collected one-dimensional raster-scan observations of the active star-forming region Sharpless 171 (S171), a typical HII 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,  52 μm,  57 μm,  63 μm,  88 μm,  122 μm,  146 μm,  158 μm, and  35 μm together with the far-infrared continuum and the H2 pure rotation transition (–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  146 μm emission was observed and the  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  63 μm and the  158 μm emission by overlapping PDRs along the line of sight can account for the observations and that the  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  35 μm emission is quite strong at almost all the observed positions. The correlation with  122 μm suggests that the  emission originates mostly from the ionized gas. The  35 μm to  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.
Key words: infrared: ISM: lines and bands / HII regions / PDR / ISM: individual objects: Sharpless 171
© ESO, 2003
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