Optical studies of Cassiopeia A. VI - Observations during the period 1976-1980

Abstract

Previously published observations of the fast moving knots in Cas A obtained between 1951 and 1975 have been supplemented by eight new Hale 5 m prime focus plates taken in 1976, 1977, and 1980. Combining new observations of 46 long-lived knots with previously published data yields an explosion date T<SUB>0</SUB> = 1658 + for - 3. No significant deceleration was detected in 36 knots that have been observed for at least 20 years. New proper motions are also given for three quasi-stationary flocculi. Attention is drawn to a cluster of exceedingly faint flocculi located outside the Cas A radio shell. One of these objects became visible almost simultaneously at optical land at radio wavelengths.