Mid- to far-infrared spectroscopy of Sharpless 171^*

¹ 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, okada@astron.s.u-tokyo.ac.jp

Received: 7 October 2002
Accepted: 11 September 2003

Abstract

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.