Ejection of Fragments in Supernova Explosions
  Recent observations by the ROSAT X-ray satellite of the Vela supernova remnant have revealed, in addition to the previously identified compact nebula, a nearly circular emitting region with a radius of about 4 degrees. The Vela pulsar is slightly off the center of this circular region, consistent with its measured proper motion of about 100km/s and an age of about 10^4 yr. The emitting region is bounded by the main supernova shock. Just outside the shock, the X-ray image reveals several well-defined V-shaped features extending radially outwards. These features are most likely wakes produced by objects moving supersonically through the outside medium. The shapes and orientations of the wakes suggest that these objects have been ejected from the center of the supernova explosion. Their present positions indicate that they have been moving with a mean velocity of a few thousand km/s. We show that pre-existing objects, such as planets in orbit around the progenitor star, could not have been accelerated to sufficiently high velocities or would have been destroyed. Instead, we propose that the observed objects are fragments ejected during the formation of the neutron star. Fragmentation during gravitational collapse is a natural consequence of both convective and rotational instabilities. 
A. Loeb, F.A. Rasio, and J. Shaham