The X-ray, optical and radio properties of young supernova remnants

The expansion of hot gas spheres into a cold uniform medium is studied as a model for the evolution of young supernova remnants. The formation of fast optical filaments in a young supernova is attributed to a Rayleigh–Taylor instability as the expanding sphere sweeps up interstellar material. In the expansion, energy is stored in the velocity dispersion of the ejecta and this is made available in the form of internal energy and random motions as the remnant decelerates. Detailed hydrodynamical calculations lead to an estimate of the amount of energy that becomes available and to prediction of the properties of filaments. During the process of filament formation, the ejecta are re-heated to a high temperature as they coalesce to form a thin shell. The thermal X-ray emission of this hot gas can account for the observed X-ray luminosity of Cas A. Secondary instabilities as the filaments disperse provide a natural way of producing a shell of magnetic field in young remnants. It appears that many of the properties of young supernova remnants can be attributed to liberation of this energy stored in the velocity dispersion of the ejecta.