The Expansion Asymmetry and Age of the Cassiopeia A Supernova Remnant

HST ACS images of the young SN remnant Cas A are used to explore theexpansion and spatial distribution of its highest velocity debris. Propermotions of over 1800 outlying ejecta knots are reported. The distribution oftransverse expansion velocities for these knots shows a striking bipolarasymmetry with the highest velocity knots confined to nearly opposing northeastand southwest `jets'. The jets appear kinematically and chemically distinctwith respect to the remnant's highest velocity debris seen in other directions.Significant gaps in the spatial distribution of outlying ejecta lie indirections which are approximately perpendicular to the jets. Extrapolations of9 month proper motions for all outer ejecta knots and a subsample of 72 brightand compact knots suggest explosion dates (assuming no knot deceleration) of1662 +/- 27 and 1672 +/- 18, respectively. We find some evidence fornon-uniform deceleration in different directions with knots located along thenorthwestern limb among the least decelerated ejecta suggesting a convergencedate of 1681 +/-19. The remnant's central X-ray point source lies some $7''$ tothe southeast of the estimated expansion center (PA = 169 deg) indicating aprojected motion of ~350 km/s toward the middle of the broad southern outerejecta knot gap.Comment: 13 pages, 5 figures, ApJ, in pres