Supernova Remnants in Molecular Clouds

Molecular clouds are known to be clumpy, with dense molecular clumpsoccupying only a few percent of the volume. A supernova remnant then evolvesprimarily in the interclump medium, and becomes radiative at a radius of about6 pc, forming a shell that is magnetically supported. When this shell interactswith the dense clumps, the molecular shock fronts are driven by a considerableoverpressure compared to the pressure in the rest of the remnant. Observationsof the remnants W44 and IC 443 can be understood in this model. Both remnantsare shell sources of radio synchrotron emission, which can be attributed torelativistic electrons in the cool radiative shell. If ambient cosmic rayelectrons are accelerated by the shock front and by the postshock compression,the radio fluxes and the flat spectral indices of W44 and IC 443 can beexplained, as well as the high energy gamma ray emission from bremsstrahlung.Comment: 31 pages, 4 figures, revised, in press, ApJ, Feb. 1, 199