Shell Model for Cassiopeia A
We model the Cassiopeia A supernova remnant in the framework of the
circumstellar medium (CSM) interaction picture (Chevalier & Liang
1989). In this model, the slow red supergiant wind of the supernova
(SN) progenitor was swept into a dense shell by a fast stellar wind in
the subsequent blue supergiant stage of the progenitor star.
The supernova blast wave propagated quickly (less than 100 years) through
the tenuous wind-blown bubble located within this shell, and then
slowed down in the dense CSM shell. The
shell was impulsively accelerated during this interaction stage; during
the subsequent interaction with SN ejecta, the shell has been further
accelerated to approximately 2000 km/s, the currently observed
expansion rate.