Bullets in a Core Collapse Supernova Remnant: The Vela Remnant

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, $\sim 0.1$ 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 $\chi \sim 100$ 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, $\chi \sim 1000$ may be required. In this case, the clump should be near the inflection point in the ejecta density profile, at an ejecta velocity $\sim 3000 \kms$. 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.