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 ~ { } 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 .