Evolution of nonradiative supernova remnants

We conduct an analytic and numerical study of the dynamics of supernova rem nant (SNR) evolution from the ejecta-dominated stage through the Sedov-Tayl or (ST) stage, the stages that precede the onset of dynamically significant radiative losses and/or pressure confinement by the ambient medium. We ass ume spherical symmetry and focus on the evolution of ejecta described by a power-law density distribution expanding into a uniform ambient medium. We emphasize that all nonradiative remnants of a given power-law structure evo lve according to a single unified solution, and we discuss this general pro perty in detail. Use of dimensionless quantities constructed from the chara cteristic dimensional parameters of the problem-the ejecta energy, ejecta m ass, and ambient density-makes the unified nature of the solution manifest. It is also possible to obtain a unified solution for the ST and radiative stages of evolution, and we place our work in the context of scaling laws f or solutions for SNR evolution in those stages. We present numerical simula tions of the flow and approximate analytic solutions for the motions of bot h the reverse shock and blast-wave shock. These solutions follow the shocks through the nonradiative stages of remnant evolution across periods of sel f-similar flow linked by non-self-similar behavior. We elucidate the depend ence of the ejecta-dominated evolution on the ejecta power-law index n by d eveloping a general trajectory for all n and explaining its relation to the solutions of Chevalier and Nadyozhin for n > 5 and Hamilton & Sarazin for n = 0. We demonstrate excellent agreement between our analytic solutions an d numerical simulations. These solutions should be valuable in describing r emnants such as SN 1006, Tycho, Kepler, Cassiopeia A, and other relatively young SNRs that are between the early ejecta-dominated stage and the late S edov-Taylor stage. In appendices, we extend our results to power-law ambien t media, and we describe an early period of the evolution in which the SNR is radiative and evolves according to a nonunified solution.