Explosive nucleosynthesis in massive stars: Comparison with the Cassiopeia a fast-moving knots

If the ejecta of a Type II supernova do not undergo extensive mixing, based on the explosion of current presupernova models, only a small fraction < or approx. =0.1 M/sub sun/ of the mantle of a massive star can yield abundances similar to those observed in the fast-moving knots of Cas A. This is shown to be independent of the detailed structure of the mantle and the supernova energy. Lack of mixing of Cas A is indicated by strong upper limits on the abundance ratios Ne/O, Mg/O, and Fe/O. If this is confirmed by further observations, then either Cas A is not the result of a standard progenitor of < or approx. =25 M/sub sun/ disrupted by a Type II supernova, or our picture of the late stages of stellar evolution in massive stars needs to be modified substantially