The observed shock wave positions and expansion in Cas A can be interpreted in a model of supernova interaction with a freely expanding stellar wind with a mass loss rate of \sim 3 \times 10 ^ { -5 } ~ { } ~ { } M _ { \odot } ~ { } yr ^ { -1 } 
for a wind velocity of 10 ~ { } km~ { } s ^ { -1 } .
The wind was probably still being lost at the time of the supernova , which may have been of Type IIn or IIb .
The wind may play a role in the formation of very fast knots observed in Cas A .
In this model , the quasi-stationary flocculi ( QSFs ) represent clumps in the wind , with a density contrast of several 10 ^ { 3 } compared to the smooth wind .
The outer , unshocked clumpy wind is photoionized by radiation from the supernova , and is observed as a patchy HII region around Cas A .
This gas has a lower density than the QSFs and is heated by nonradiative shocks driven by the blast wave .
Denser clumps have recombined and are observed as HI compact absorption features towards Cas A .