Optical studies of Cassiopeia A. VII - Recent observations of the structure and evolution of the nebulosity

Abstract

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.