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

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