The Interaction of Type Ia Supernovae with their Surroundings: The   Exponential profile in 2D
  The evolution of Type Ia supernovae in the surrounding medium is studied using 2-dimensional numerical hydrodynamic simulations. The ejecta are assumed to be described by an exponential density profile, following the work of Dwarkadas & Chevalier (1998). The case of a circumstellar (CS) region formed by mass loss from the progenitor or a companion star is also considered. The decelerating contact discontinuity is found to be Rayleigh-Taylor (R-T) unstable, as expected, and the nature of the instability is studied in detail for 2 cases: 1) a constant density ambient medium, and 2) a CS medium whose density goes as r^{-2}. The nature of the instability is found to be different in both cases. In the case of a CS medium the instability is much better resolved, and a fractal-like structure is seen. In the case of a constant density medium the extent of growth is less, and the R-T fingers are found to be limited by the presence of Kelvin-Helmholtz mushroom caps at the tips of the fingers. The unstable region is far enough away from the reverse shock that the latter is not affected by the mixing taking place in the interaction region. In contrast the reverse shock in the case of a CS medium is found to be rippled due to the formation of instabilities. In neither case is the outer shock front affected. These results are consistent with similar studies of power-law ejecta profiles conducted by Chevalier, Blondin & Emmering (1992). Our results are then applied to Tycho's supernova remnant. 
Vikram V. Dwarkadas (RCfTA, Univ of Sydney)