HST images of the young supernova remnant Cas A are used to explore the expansion and spatial distribution of its highest velocity debris .
ACS/WFC images taken in March and December 2004 with Sloan F625W , F775W , and F850LP filters were used to identify 1825 high-velocity , outlying ejecta knots through measured proper motions of 0 \hbox { $ . ^ { \prime \prime } $ } 35 - 0 \hbox { $ . ^ { \prime \prime } $ } 90 yr ^ { -1 } corresponding to V _ { trans } = 5500 - 14500 km s ^ { -1 } assuming d = 3.4 kpc .
The distribution of derived transverse expansion velocities for these ejecta knots shows a striking bipolar asymmetry with the highest velocity knots ( V _ { trans } \geq 10500 km s ^ { -1 } ) confined to nearly opposing northeast and southwest ‘ jets ’ at P.A .
= 45 { \hbox { $ { } ^ { \circ } $ } } -70 { \hbox { $ { } ^ { \circ } $ } } and 230 { \hbox { $ { } ^ { \circ } $ } } -270 { \hbox { $ { } ^ { \circ } $ } } , respectively .
The jets have about the same maximum expansion velocity of \simeq 14000 km s ^ { -1 } and appear kinematically and chemically distinct in that they are the remnant ’ s only S-rich ejecta with expansion velocities above the 10000 - 11000 km s ^ { -1 } exhibited by outer nitrogen-rich ejecta which otherwise represent the remnant ’ s highest velocity debris .
In addition , we find significant gaps in the spatial distribution of outlying ejecta in directions which are approximately perpendicular to the jets ( P.A .
= 145 { \hbox { $ { } ^ { \circ } $ } } -200 { \hbox { $ { } ^ { \circ } $ } } and 335 { \hbox { $ { } ^ { \circ } $ } } -350 { \hbox { $ { } ^ { \circ } $ } } ) .
The remnant ’ s central X-ray point source lies some 7 ^ { \prime \prime } to the southeast of the estimated expansion center ( PA = 169 { \hbox { $ { } ^ { \circ } $ } } \pm 8.4 { \hbox { $ { } ^ { \circ } $ } } ) indicating a projected motion toward the middle of the broad southern ejecta knot gap .
Extrapolations of measured nine month proper motions for all 1825 outer ejecta knots and a selected subsample of 72 bright and compact knots suggest explosion dates ( assuming no knot deceleration ) of 1662 \pm 27 and 1672 \pm 18 , respectively .
We find some evidence for non-uniform deceleration in different directions around the remnant and find 126 knots located along the northwestern limb among the least decelerated ejecta suggesting a convergence date of 1681 \pm 19 .
A remnant age of around 325 yr would imply a \simeq 350 km s ^ { -1 } transverse velocity for the central X-ray point source .