Bullets in a Core‐Collapse Supernova Remnant: The Vela Remnant

We use two-dimensional hydrodynamical simulations to investigate theproperties of dense ejecta clumps (bullets) in a core collapse supernovaremnant, motivated by the observation of protrusions probably caused by clumpsin the Vela supernova remnant. The ejecta, with an inner flat and an outersteep power law density distribution, were assumed to freely expand into anambient medium with a constant density, $\sim 0.1$ H atoms cm$^{-3}$ for thecase of Vela. At an age of $10^4$ yr, the reverse shock front is expected tohave moved back to the center of the remnant. Ejecta clumps with an initialdensity contrast $\chi \sim 100$ relative to their surroundings are found to berapidly fragmented and decelerated. In order to cause a pronounced protrusionon the blast wave, as observed in the Vela remnant, $\chi \sim 1000$ may berequired. In this case, the clump should be near the inflection point in theejecta density profile, at an ejecta velocity $\sim 3000 \kms $. These resultsapply to moderately large clumps; smaller clumps would require an even largerdensity contrast. Clumps can create ring structure in the shell of the Velaremnant and we investigate the possibility that RX J0852--4622, an apparentsupernova remnant superposed on Vela, is actually part of the Vela shell. Radioobservations support this picture, but the possible presence of a compactobject argues against it. The Ni bubble effect or compression in a pulsar windnebula are possible mechanisms to produce the clumping.Comment: 3 PS files, 3 GIF files, Latex, submitted to Ap