The Evolution of Supernova Remnants. Spherically Symmetric Models

The evolution of supernova remnants is studied using a spherically symmetric hydrodynamic code with a magnetic field approximately included. Properties of the emitted radiation are investigated. Particular attention is paid to the late phases when a dense neutral shell forms which accretes matter from both the interstellar medium and the hot interior. The shell absorbs hard ultraviolet and soft X-ray photons. The interstellar magnetic field produces the dominant pressure in the shell and determines its density and thickness. Results for explosions of 3 x 1050 ergs in gas of densities 10-2, 100, and 10 show that harder radiation emerges from explosions in higher density gas. Using a scaling law, the results can be extended to a range of densities and initial energies. If cooling by collisionally heated grains occurs in the high-density case, about 20 percent of the total energy is radiated in the infrared in the early stages. In later phases, infrared cooling by radiation heated grains and infrared line emission may be significant, particularly in the low-density case. At the end of a typical run, the integrated radiated energy is about equally divided between infrared and ionizing radiation. Interaction of the supernova shock with a large cloud decreases the total kinetic energy, and increases the thermal energy due to the reflected shock. The possibility that the initial supernova energy is deposited as relativistic particles is investigated; it is found that the shock radius a few thousand years after the explosion is fairly independent of the initial conditions, while differences when the radius is of order 1 pc are considerable. Comparison is made with observations. The observed relation between the surface brightness and diameter of old supernova remnants can be understood on the basis of the present models and van der Laan's theory for the radio emission. Subject headings: hydrodynamics - supernovae - supernova remnants