Optical Studies of Cassiopeia a. I. Proper Motions in the Optical Remnant

Abstract

Sixteen 200-inch plates taken between 1951 and 1968 have been used to study the expansion of the optical remnant of Cas A. If it is assumed that no deceleration occurred, the explosion of this supernova took place in A.D. 1667 + 8 (m.e.). The position of the center of expansion is found to be ai = l 0.2, = +58032i18.9 + 3'.'1. No star brighter than V = 22.5 is visible within 8 standard deviations of this position. If values of A = 6.8 and D = 2.8 kpc are assumed, it follows that any remaining stellar remnant must be fainter than Al7 = +3.5. The optical remnant of Cas A is undergoing rapid changes. The average decay time scale for moving knots is found to be about 10 years.