Optical studies of Cassiopeia A. VII. Recent observations of the structure and evolution of the nebulosity.
Palomar observations obtained between 1951 and 1983 are used to study the evolution of Cas A. Most quasi-stationary flocculi (QSFs) are found to have lifetimes greater than about 25 yr. Those QSFs outside the main radio shell appear to have below-average lifetimes. It is suggested that tadpole-like QSFs may be older than more compact flocculi situated in regions that presently contain fast-moving knots. High-excitation fast--moving knots, which radiate in forbidden O III, seem to be somewhat more ephemeral than knots that radiate in forbidden S II. Attention is drawn to two features (R35 and the base of the jet) which appear to have some characteristics that are intermediate between those of QSFs and fast-moving knots. The observation that the highest velocity knots emit forbidden S II but not forbidden O III suggests that the supernova precursor was not well mixed. New observations of quasi-stationary flocculi yield an expansion age of 11,000 + or - 2000 yr.