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 \alpha ( J2000 ) = 23 ^ { h } 23 ^ { m } 27 \fs 77 \pm 0 \fs 05 , \delta ( J2000 ) = 58 ^ { o } 48 ^ { \prime } 49 \farcs 4 \pm 0 \farcs 4 ( ICRS ) , with little difference between centers derived using outer or main shell knots .
This position is 3 \farcs 0 due north of that estimated by van den Bergh & Kamper ( 41 ) .
It also lies 6 \farcs 6 \pm 1 \farcs 5 almost due north ( PA = 354 ^ { \circ } ) of the remnant ’ s recently-detected central X-ray point source , implying a transverse velocity for the X-ray point source \simeq 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 \pm 0.9 assuming no deceleration .
However , a deceleration of just \sim 1.6 km s ^ { -1 } yr ^ { -1 } over a 300 yr time span would produce an explosion date \simeq A.D. 1680 , consistent with the suspected sighting of the Cas A supernova by J. Flamsteed .