We present two-dimensional high-resolution hydrodynamical simulations in spherical polar coordinates of a Type Ia supernova interacting with a constant-density interstellar medium. The ejecta are assumed to be freely expanding with an exponential density proÐle. The interaction gives rise to a double-shocked structure susceptible to hydrodynamical instabilities. The Rayleigh-Taylor instability initially grows, but the Kelvin-Helmholtz instability takes over, producing vortex rings. Provided the simulation is initiated early in the evolution, with a perturbation the instability reaches its fully Z1%, developed nonlinear strength within 5 doubling times. The further nonlinear evolution does not depend on the initial conditions. Considering the small initial radii of Type Ia supernovae, they are likely to reach this fully developed phase. The nonlinear instability initially evolves toward longer wavelengths and eventually fades away when the reverse shock front is in the Ñatter part of the supernova density distribution. On the basis of observations of X-ray knots and the protrusion in the southeast outline of TychoÏs supernova remnant, we include clumping in the ejecta because these features cannot be explained by instabilities growing from small perturbations. The clump interaction with the reverse shock induces Rayleigh-Taylor and Kelvin-Helmholtz instabilities on the clump surface that facilitate fragmentation. In order to survive crushing and to have a bulging e †ect on the forward shock, the clumpÏs initial density ratio to the surrounding ejecta must be at least 100 for the conditions in TychoÏs remnant. The 56Ni bubble e †ect may be important for the development of clumpiness in the ejecta. The observed presence of an Fe clump would then require a nonradioactive origin for this Fe, possibly 54Fe. The large radial distance of the X-rayÈemitting Si and S ejecta from the remnant center indicates that they were initially in clumps.

We use two-dimensional hydrodynamical simulations to investigate the properties of dense ejecta clumps (bullets) in a core collapse supernova remnant, motivated by the observation of protrusions probably caused by clumps in the Vela supernova remnant. The ejecta, with an inner flat and an outer steep power law density distribution, were assumed to freely expand into an ambient medium with a constant density, ∼ 0.1 H atoms cm −3 for the case of Vela. At an age of 10 4 yr, the reverse shock front is expected to have moved back to the center of the remnant. Ejecta clumps with an initial density contrast χ ∼ 100 relative to their surroundings are found to be rapidly fragmented and decelerated. In order to cause a pronounced protrusion on the blast wave, as observed in the Vela remnant, χ ∼ 1000 may be required. In this case, the clump should be near the inflection point in the ejecta density profile, at an ejecta velocity ∼ 3000 km s −1 . These results apply to moderately large clumps; smaller clumps would require an even larger density contrast. Clumps can create ring structure in the shell of the Vela remnant and we investigate the possibility that RX J0852-4622, an apparent supernova remnant superposed on Vela, is actually part of the Vela shell. Radio observations support this picture, but the possible presence of a compact object argues against it. The Ni bubble effect or compression in a pulsar wind nebula are possible mechanisms to produce the clumping.

We used one-dimensional radiative-transport radiation hydrodynamical (RHD) simulations to investigate the formation of clumping in freely-expanding supernova ejecta due to the radioactive heating from the Ni56 -> Co56 -> Fe56 decay sequence. The heating gives rise to an inflated Nickel bubble, which induces a forward shock that compresses the outer ambient gas into a shell. The radiative energy deposited by the radioactivity leaks out across the shock by radiative diffusion, and we investigate its effect on the evolution of the ejecta structure. Compared to the hydrodynamical adiabatic approximation with gamma =4/3, the preshock gas becomes accelerated by the radiation outflow. The shock is thus weakened and the shell becomes broader and less dense. The thickness of the shell takes up <~ 4 % of the radius of the bubble, and the structure of the shell can be approximately described by a self-similar solution. We compared the properties of the shell components with those of the ejecta clumps indicated by our previous hydrodynamical simulations for the later interaction of clumps with the outer supernova remnant. The high density contrast across the shell, chi ~ 100, is compatible with that of ejecta clumps as indicated for Tycho's knots, but there is insufficient dense gas to cause a pronounced protrusion on the outline of a core collapse supernova remnant, like the bullets in the Vela remnant.Comment: accepted by Ap

This study investigates the evolution of Rayleigh–Taylor (R–T) instabilities in Type Ia supernova remnants that are associated with a low adiabatic index γ, where γ < 5/3, which reflects the expected change in the supernova shock structure as a result of cosmic ray particle acceleration. Extreme cases, such as the case with the maximum compression ratio that corresponds to γ= 1.1, are examined. As γ decreases, the shock compression ratio rises, and an increasingly narrow intershock region with a more pronounced initial mixture of R–T unstable gas is produced. Consequently, the remnant outline may be perturbed by small‐amplitude, small‐wavelength bumps. However, as the instability decays over time, the extent of convective mixing in terms of the ratio of the radius of the R–T fingers to the blast wave does not strongly depend on the value of γ for γ≥ 1.2. As a result of the age of the remnant, the unstable gas cannot extend sufficiently far to form metal‐enriched filaments of ejecta material close to the periphery of Tycho’s supernova remnant. The consistency of the dynamic properties of Tycho’s remnant with the adiabatic model γ= 5/3 reveals that the injection of cosmic rays is too weak to alter the shock structure. Even with very efficient acceleration of cosmic rays at the shock, significantly enhanced mixing is not expected in Type Ia supernova remnants.