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Nucleosynthesis and Mixing in Cassiopeia A

We present results from the first light observations of the Cassiopeia A (Cas A) supernova remnant (SNR) by the Chandra X-ray Observatory. The X-ray spectrum varies on all spatial scales down to the instrumental limit (0.02 pc at the SNR). Based on representative spectra from four selected regions we investigate the processes of nucleosynthesis and mixing in Cas A. We make the first unequivocal identification of iron-rich ejecta produced by explosive silicon-burning in a young Galactic SNR. Elsewhere in the remnant we see silicon-rich ejecta from explosive oxygen-burning. Remarkably, our study finds that the Fe-rich ejecta lies outside the Si-rich material, leading to the conclusion that bulk motions of the ejecta were extensive and energetic enough in Cas A to cause a spatial inversion of a significant portion of the supernova core during the explosion. It is likely that this inversion was caused by ``Fe''-rich ejecta emerging in plumes from the rising bubbles in the neutrino-driven convection layer. In addition the radioactive decay energy from $^{56}$Ni may have contributed to the subsequent evolution of the material. We have also discovered faint, well-defined filaments with featureless X-ray spectra that are possibly the sites of cosmic ray acceleration in Cas A.