Far-Infrared Spectroscopy of Diffuse Interstellar Medium - Large Silicon Abundance and Effects of UV Radiation

¹ Department of Astronomy, University of Tokyo, Tokyo 113-0033, Japan
² Central Research Laboratory, Hitachi Ltd., Tokyo 185-8601, Japan (present address)
³ Department of Earth Science and Astronomy, University of Tokyo, Tokyo 153-8902, Japan
⁴ Department of Physics, Nagoya University, Nagoya 464-8602, Japan

Corresponding author: onaka@astron.s.u-tokyo.ac.jp

Abstract

The results of FIR spectroscopic mapping observations of the Carina and Sharpless 171 regions with ISO/LWS and SWS are presented. Quite strong [SiII]35 μm emission has been detected in both regions. It is well correlated with [NII]122 μm emission, indicating that it originates mostly from low-density ionized gas. The ratio of the lines shows that S⁺ gas must be more than 35% of the solar abundance in the ionized medium, suggesting clear observational evidence for the dust destruction in the interstellar medium. We have also found a small line ratio of [OI]63 μm to 145 μm lines (<10) in both regions. The observed large ratio as well as their spatial distribution cannot be accounted for by standard PDR models, even if taking account of the pumping by ultraviolet radiation. We propose that [OI]63 μm line is absorbed in the cold gas between the line emitting region and us.