Nucleosynthetic Layers in the Shocked Ejecta of Cassiopeia A

TL;DR

This study provides a detailed 3D analysis of Cassiopeia A's ejecta, revealing the structure and velocity distribution of nucleosynthetic layers and their relation to supernova explosion models.

Contribution

It offers the first high-resolution 3D mapping of nucleosynthetic layers in Cassiopeia A using Spitzer spectra, constraining supernova explosion models.

Findings

The reverse shock is spherical within 7%.

Velocity width of layers is about 1000 km/s overall.

Si and O layers are sometimes coincident, sometimes segregated.

Abstract

We present a 3-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 approximately 1000 km/s over 4000 square arcsecond 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 approximately 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.