Highest velocity ejecta of Cassiopeia A

CCD interference-filter images and spectroscopy of Cassiopeia A's outlying optical emission knots are presented. Six new high-velocity forbidden N II-emitting knots are identified, which increases the total now known to 15. New spectra of seven of these knots suggest high N abundances. Spectra, radial velocity, and proper motion data indicate that these outlying knots possess ejection velocities between 7000 and 8600 km/s and are probably material from the progenitor's outer layers. These observations suggest that the Cas A supernova had a maximum expansion velocity of at least 8500 km/s, a substantial nitrogen-rich but hydrogen-deficient envelope, an explosion date around A.D. 1680, and a W progenitor. Cas A's northeastern emission jet, which consists of sulfur-rich ejecta, appears to be an isolated, well-collimated structure extending at least 265 arcsec from the remnant center. Such radial distances imply ejection velocities of up to 11,000 km/s and at least a partially asymmetric stellar disruption.