Evolution of supernova remnants. IV. The supernova ejecta

A model for Cas A based on the properties of the fast moving knots is investigated. The knots form during or soon after supernova explosion. They become immersed in hot gas, which heats the knots by thermal conduction provided radiative cooling by overabundant heavy elements is not too strong. The heating time scale is of the same order as or less than the age of Cas A. As a supernova remnant evolves, both the heated ejecta and the interstellar medium emit X-rays. The central X-ray source in the Cygnus Loop may be heated ejecta. The relevance of the model for other supernova remnants is considered. It is likely that the supernova ejecta are eventually mixed through the hot, low-density interior of a supernova remnant. Cooling of the hot gas can occur on a time scale of order 105--106 years because of the heating, radiative, and dynamical effects due to conduction at the interface with an interstellar cloud. The effect of the expansion of a young remnant on oxygen recombination is investigated and found to be important at low densities. Such effects must be taken into account in attempting to derive abundances from X-ray lines, once they can be measured observationally