Particle representations of the spatial distributions of He, C, O, and Ni+X for models W15-2-cw, L15-1-cw, N20-4-cw, and B15-1-pw (from top) at the time of shock breakout. We show only particles within a slab given by . The dashed lines give the shock’s location in the y = 0 plane. The particles are colored according to their radial velocity, which is binned into four bins, each bin containing 25% of the total mass of the element. Blackish particles represent the slowest 25% by mass of an element, while bluish particles mark the fastest 25%. The black thin shell contains matter that was hit by the reverse shock, and thus moves slower than matter located inside this shell (reddish and greenish particles). The positive y-axis points away from the viewer.