A circumstellar shell model for the Cassiopeia A supernova remnant

We model the Cassiopeia A supernova remnant in the framework of the circumstellar medium (CSM) interaction picture. 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 or equal to 100 yr) through the tenuous wind-blown bubble located within this shell and then slowed down in the dense (n(H) similar to 15 cm(-3)) 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 similar to 2000 km s(-1), the currently observed expansion rate. The comparison of our X-ray emission calculations with the ASCA spectrum suggests that about 8 M. of X-ray-emitting material is present in Cas A. Most of this mass is located in the CSM shell and in the outlying red supergiant wind. The X-ray continuum and the Fe K alpha line are dominated by the shell emission, but prominent K alpha complexes of Mg, Si, and S must be produced by SN ejecta with strongly enhanced abundances of these elements. Our hydrodynamical models indicate that about 2 M. of ejecta have been shocked. An explosion of a stellar He core is consistent with these findings.