Cassiopeia A and Its Clumpy Presupernova Wind

The observed shock wave positions and expansion in Cassiopeia A can be interpreted in a model of supernova interaction with a freely expanding stellar wind with a mass loss rate of ~2 × 10 -5 M ☉ 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 Type 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 with the smooth wind. The outer, unshocked clumpy wind is photoionized by radiation from the supernova and is observed as a patchy H II 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 H I compact absorption features toward Cas A.