2012ApJ...757..126I

We present a three-dimensional analysis of the supernova remnant Cassiopeia A using high-resolution spectra from the Spitzer Space Telescope. We observe supernova ejecta both immediately before and during the shock-ejecta interaction. We determine that the reverse shock of the remnant is spherical to within 7%, although the center of this sphere is offset from the geometric center of the remnant by 810 km/s. We determine that the velocity width of the nucleosynthetic layers is ∼1000 km/s over 4000 arcsec² regions, although the velocity width of a layer along any individual line of sight is <250 km/s. Si and O, which come from different nucleosynthetic layers in the progenitor star, are observed to be coincident in velocity space in some directions, but segregated by up to ∼500 km/s in other directions. We compare these observations of the nucleosynthetic layers to predictions from supernova explosion models in an attempt to constrain such models. Finally, we observe small-scale, corrugated velocity structures that are likely caused during the supernova explosion itself, rather than hundreds of years later by dynamical instabilities at the remnant's reverse shock.