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Session 18 - The Interstellar Medium.
Oral session, Monday, June 08
We present Infrared Space Observatory spectroscopy of a site in the supernova remnant 3C 391, where the blast wave is impacting a dense clump in a giant molecular cloud. The complete wavelength ranges of 40--190 (LWS) and 5--17 (CAM) \mum were observed, as well as narrow ranges centered on 26 and 34.8 (SWS) \mum. Atomic fine-structure lines were detected from (in order of atomic number): C^+, N^+, N^++, O^0, O^++, Ne^+, Ne^++, Si^+, P^+, Ar^+, and Fe^+. The presence of a wide range of abundant ions indicates a strong, dissociative (J-type) shock into the lower-density portions of the cloud. The [P II] line at 60.6 \mum is the first known astronomical detection. We discuss the likelihood of this detection in terms of the atomic structure and interstellar abundance of phosphorus. Molecular rotational lines were detected from H_2, CO, OH, and H_2O. All lines arise from low-lying energy levels, consistent with collisional excitation in the dense and warm post-shock gas. Together with millimeter-wave lines of CO, CS, and HCO^+, the infrared molecular lines are more consistent with a C-type or partially dissociative shock into a region of very high preshock density. The presence of both J-type and C-type shocks in this small region show that the pre-shock medium had a wide range of densities: dense pre-shock clumps (10^5 cm^-3) lead to the shock-excited molecular lines, while a more tenuous medium (10^3 cm^-3) leads to the shock-excited atomic fine-structure lines.
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