The Circumstellar Medium of Cassiopeia a Inferred from the Outer Ejecta Knot Properties

We investigate the effect of the circumstellar medium density profile on the X-ray emission from outer ejecta knots in the Cassiopeia A supernova remnant using the 1 Ms Chandra observation. The spectra of a number of radial series of ejecta knots at various positions around the remnant are analyzed using techniques similar to those devised in previous papers. We can obtain a reasonable match to our data for a circumstellar density profile proportional to r ^-2 as would arise from the steady dense wind of a red supergiant, but the agreement is improved if we introduce a small (0.2-0.3 pc) central cavity around the progenitor into our models. Such a profile might arise if the progenitor emitted a fast tenuous stellar wind for a short period immediately prior to explosion. We review other lines of evidence supporting this conclusion. The spectra also indicate the widespread presence of Fe-enriched plasma that was presumably formed by complete Si burning during the explosion, possibly via alpha-rich freezeout. This component is typically associated with hotter and more highly ionized gas than the bulk of the O- and Si-rich ejecta.