The Expansion Center and Dynamical Age of the Galactic Supernova Remnant Cassiopeia A^*

We present proper motions for 21 bright main shell and 17 faint, higher velocity, outer ejecta knots in the Cas A supernova remnant and use them to derive new estimates for the remnant's expansion center and age. Our study included 1951–1976 Palomar 5 m prime focus plates, 1988–1999 CCD images from the KPNO 4 m and MDM 2.4 m telescopes, and 1999 HST WFPC2 images. Measurable positions covered a 23 to 41 yr time span for most knots, with a few outer knots followed for almost 48 yr. We derive an expansion center of α(J2000) = 23^h23^m2777 ± 005, δ(J2000) = 58°48'494 ± 04 (ICRS), with little difference between centers derived using outer or main shell knots. This position is 30 due north of that estimated by van den Bergh & Kamper. It also lies 66 ± 15 almost due north (P.A. = 354°) of the remnant's recently detected central X-ray point source, implying a transverse velocity for the X-ray point source ≃330 km s^-1 at a distance of 3.4 kpc. Using the knots which lie out ahead of the remnant's forward blast wave, we estimate a knot convergent date of A.D. 1671.3 ± 0.9 assuming no deceleration. However, a deceleration of just ∼1.6 km s^-1 yr^-1 over a 300 yr time span would produce an explosion date ≃A.D. 1680, consistent with the suspected sighting of the Cas A supernova by J. Flamsteed.