Shocked molecular hydrogen in the supernova remnant IC 443

Emission from the υ = 1–0 S(1) line of molecular hydrogen has been mapped over a section of the supernova remnant IC443. The emission originates in a sinuous ridge where the expanding shell of the SNR is interacting with a molecular cloud. The relative intensities of the 1–0 S(1), 1–0 S(0) and 2–1 S(1) lines at 2.1–2.2μm were measured and found to be characteristic of shock-excitation of the gas. The ridge shows bright spots which are possibly density enhancements in the molecular cloud. The total luminosity of the molecular hydrogen lines in the mapped region is estimated to be about $$1000\enspace L_\odot$$, making IC443 one of the most luminous galactic molecular hydrogen sources yet detected. The morphology of the S(1) line emission is similar to that of the millimetre emission lines from CO, HCO⁺ and HCN and to that of the 21-cm emission line from atomic hydrogen. There is evidence for partial dissociation of molecular hydrogen caused by the shock interaction, but the absence of Brγ line emission puts a low upper limit on the degree of ionization. The energetics of the SNR are consistent with most of the mechanical energy incident on the molecular cloud being radiated away through H₂ lines and/or being used to dissociate the H₂, except in the densest clumps (where shocks are either slow or non-existent), and where far-infrared emission from collisionally heated grains may provide the dominant cooling mechanism. A model for IC 443 is presented involving the expansion of the SN shock-wave inside the remains of a molecular disc left over from the process of formation of the slar which exploded.