Young supernova remnants in nonuniform media

We investigate the dynamics of young supernova remnants (SNRs) in the presence of a nonuniform external density such as might be present at the edge of a molecular cloud. Using a two-dimensional, Total Variation Diminishing, hydrodynamic code, we perform high-resolution (1024 by 2048) simulations in cylindrical symmetry of explosions in the vicinity of a smooth density transition. We improve upon previous calculations by including the inertia and kinetic energy of the stellar ejecta. The evolution is followed until the outer blast wave has swept up roughly 20 times the initial ejected mass (14.5 M.). We find that the presence of the density transition has important dynamical effects on the remnant during the pre-Sedov-Taylor evolution. When the remnant is smaller than the width of the density transition, the apparent SNR center shifts while maintaining a nearly circular shape. At this stage, the morphology of such a SNR would be difficult to distinguish from one expanding in a uniform medium. Once the remnant size exceeds the transition width, it expands as distinct lobes into the high- and low-density regions. The radii of these lobes, as measured from the explosion center, approach the self-similar Sedov-Taylor solution appropriate for the gas density that is local to each lobe. The nonuniform density also creates a pressure imbalance in the SNR interior that drives gas from the high-density side to the low-density side. This sets up asymmetric, nonradial flows in the remnant interior, which may be directly observable. At early stages, the nonspherical motions should serve as a better indicator of a nonuniform external density than the morphology.