The death of massive stars is believed to involve aspheric explosions initiated by the collapse of an iron core. The specifics of how these catastrophic explosions proceed remain uncertain due, in part, to limited observational constraints on various
processes that can introduce asymmetries deep inside the star. Here we present near-infrared observations of the young Milky Way supernova remnant Cassiopeia A, descendant of a type IIb core-collapse explosion, and a three-dimensional map of its interior, unshocked ejecta. The remnants interior has a bubble-like morphology that smoothly connects to and helps explain the multi-ringed structures seen in the remnants bright reverse shocked main shell of expanding debris. This internal structure may have originated from turbulent mixing processes that encouraged the development of outwardly expanding plumes of radioactive 56Ni-rich ejecta. If this is true, substantial amounts of its decay product, 56Fe, may still reside in these interior cavities.
We discuss the findings of a comprehensive imaging and spectroscopic survey of the optical emission associated with the supernova remnant 3C 58 (Fesen et al. 2007) as they relate to the topic of pre-SN mass loss. Spectroscopically measured radial vel
ocities of ~450 emission knots within the remnant show two distinct kinematic populations of optical knots: a high-velocity group with radial velocities in the range of 700 - 1100 km/s and a lower velocity group exhibiting radial expansion velocities below ~250 km/s. We interpret the high-velocity knots as ejecta from the SN explosion and the low-velocity knots as shocked circumstellar material likely resulting from pre-SN mass loss. The chemical signatures of the two populations also show marked differences. The high velocity group includes a substantial number of knots with notably higher [N II]/H-alpha ratios not seen in the lower velocity population, suggesting greater nitrogen enrichment in the SN ejecta than in the CSM. These results are compared with evidence for pre-SN mass loss in the Crab Nebula, perhaps the SNR most similar to 3C 58. These SNRs may comprise two case studies of pre-SN mass loss in relatively low mass (~8 - 10 solar masses) core-collapse SN progenitors.
