We discuss ISO observations of infrared ionic and H2 lines toward molecular shocks in the supernova remnants 3C 391, W 28, and W 44. The total surface brightness of the H2 lines toward these lines of sight exceeds that of atomic fine structure lines, showing that these lines of sight are dominated by dense molecular shocks. The H2 excitation and the presence of bright ionic lines require that there are multiple shocks into gas with a range of pre-shock densities from 10–103 cm−3.

Introduction

Massive stars end their lives in supernova explosions, and they do not live long enough to travel far from their parent molecular clouds. Therefore, supernovae frequently occur inside molecular clouds, providing compression, turbulence, cosmic rays, radiation, and heat. Using the Infrared Space Observatory, we performed a set of observations designed to search for infrared emission from the gas and dust that gets excited in molecular shock fronts. When the shock front passes through a molecular cloud, the gas cools via the most ‘convenient’ transitions available to it: low-density gas cools via atomic fine structure lines from the abundant ions, while molecular gas cools via the large number of rotational and/or vibrational transitions available. The first results of our project were the detection of bright [O I] 63 µm lines (Reach & Rho 1996), proving that abundant energy was being pumped into the gas by the shock fronts.