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 (ICRS), with alpha (J2000) = 23(h)23(m)27.(s)77 +/- 0.(s)05, delta (J2000) = 58 degrees 48'49.4 +/- 0.4 little difference between centers derived using outer or main shell knots. This position is due north of 3.0 that estimated by van den Bergh & Kamper. It also lies almost due north (P.A. = 354 degrees) of the 6.6 +/- 1.5 remnant's recently detected central X-ray point source, implying a transverse velocity for the X-ray point source similar or equal to 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 similar to1.6 km s(-1) yr(-1) over a 300 yr time span would produce an explosion date similar or equal to A.D. 1680, consistent with the suspected sighting of the Cas A supernova by J. Flamsteed.