Optical studies of Cassiopeia A. V - A definitive study of proper motions

Abstract

Prime-focus plates taken with the Hale reflector during the first quarter-century of its existence have been used to derive proper motions for 102 fast-moving knots and for 30 quasi-stationary flocculi in the optical remnant of Cas A. These data yield a center of expansion for the system of fast-moving knots at a(1950) = 23h21rn12s.0 + 0s.1 and 8(1950) = + 58032'1W.9 + 0N.8 relative to the AGK3. An explosion date of A.D. 1657 + 3 is obtained from an expansion solution for all knots that remained visible for 10 or more years. Some knots that have appeared recently appear to be expanding less rapidly than predicted by the general expansion solution, whereas the knots that are located outside the shell are moving faster than expected. Explosion dates derived from the main shell of the supernova and from the knots outside this shell are A.D. 1653 and A.D. 1671, respectively. The system of quasi-stationary flocculi is found to be expanding with a time scale of 11,000 yr. This systematic motion might be due to the slow ( 150 km ) expansion of a preexisting circumstellar shell, or it might have resulted from momentum transfer by the supernova shell to stationary circumstellar material. The origin of the velocity dispersion of 3000 km 1 in filament no. 1 remains a mystery. Radial velocity observations extending over a four-year period rule out large present-day accelerations, whereas the small size of the filament militates against large accelerations that took place long ago. Finally, the absence of hydrogen lines rules out significant deceleration by hydrogen-rich interstellar clouds. The space distribution of the knots in which [0 iii] is very much brighter than [S ii] differs significantly from that for the knots in which the [0 iii]![S ii] ratio is more nearly normal. In some cases [0 i] and [0 iii] are strong in knots that do not show [S ii]. These "oxygen knots" seem to be lagging slightly behind the general expansion of the remnant. This contrasts with the behavior of the "sulfur knots" in the jet which are expanding somewhat faster than would be expected from the general expansion solution. The possibility that these effects are due to chemical stratification in the exploding supernova is discussed. Any stellar remnant at the position of the center of expansion must now be fainter than Mr = +8. Subject headings: nebulae: individual - nebulae: supernova remnants