We present a multi-wavelength study of the nearby supernova remnant Cassiopeia A (Cas A). Easily resolvable supernova remnants such as Cas A provide a unique opportunity to test supernova explosion models. Additionally, we can observe key processes in the interstellar medium as the ejecta from the initial explosion encounter Cas A’s powerful shocks.
In order to accomplish these science goals, we used the Spitzer Space Telescope’s Infrared Spectrograph to create a high resolution spectral map of select regions of Cas A, allowing us to make a Doppler reconstruction of its 3-dimensional structure structure. In the center of the remnant, we find relatively pristine ejecta that have not yet reached Cas A’s reverse shock or interacted with the circumstellar environment. We observe O, Si, and S emission. These ejecta can form both sheet-like structures as well as filaments. Si and O, which come from different nucleosynthetic layers of the star, are observed to be coincident in some regions, and separated by >500 km s−1 in others. Observed ejecta traveling toward us are, on average, #24;800 km s−1 slower than the material traveling away from us. We compare our observations to recent supernova explosion models and find that no single model can simultaneously reproduce all the observed features.
However, models of different supernova explosions can collectively produce the observed geometries and structures of the emission interior to Cas A’s reverse shock. We use the results from the models to address the conditions during the supernova explosion, concentrating on asymmetries in the shock structure. We also predict that the back surface of Cassiopeia A will begin brightening in #24;30 years, and the front surface in #24;100 years. We then used similar observations from 3 regions on Cas A’s reverse shock in order to create more 3-dimensional maps. In these regions, 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−1. We determine that the velocity width of the nucleosynthetic layers is #24;1000 km s−1 in a given region, although the velocity width of a layer along any given line of sight is <250 km s−1. Si and O are observed to be coincident in some directions, but segregated by up to #24;500 km s−1 in other directions. We again 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 velocity structures in the recently shocked ejecta. We determine that this corrugation is likely caused during the supernova explosion itself, rather than hundreds of years later at the remnant’s reverse shock.
Finally, we present a detailed multi-epoch X-ray analysis of Cas A using Chandra X-ray Observatory exposures from 2000, 2002, and 2004. We identify the most recently shocked X-ray ejecta with ionization timescales of #24;1010 cm−3 s, nearly an order of magnitude smaller than previously identified shocked ejecta. These ejecta are then used to determine if the original nucleosynthetic layers of the star are arriving at Cas A’s reverse shock at different times. We use recent collisional ionization models that allow us to correlate observed changes in spectrum with a rough estimate of when the Mg and Fe layers reached the reverse shock. We find several regions that have a signature consistent with a separation of #24;200 km s−1 between layers, although we find that most regions show no sign of separation greater than 65 km s−1. This method is able to detect substantially smaller separations between layers than earlier X-ray techniques. We test various supernova explosion models against our observations by comparing our observed velocity separation between layers to predictions from the models. We conclude that any mixing between nucleosynthetic layers is most likely caused by Rayleigh-Taylor filamentation and not partial explosive nucleosynthesis in the layers. Our observations of spectral changes provide feedback for future models which will address important physical issues such as the role of cosmic ray production at a supernova remnant’s reverse shock.