Optical spectroscopy and imaging of the northeast jet in the Cassiopeia A supernova remnant

Optical spectra for nearly 90 knots in Cas A's northeast jet, together with [S II] lambda lambda 6716, 6731 and H alpha imaging, are used to provide a detailed look at the jet's kinematic and emission-line properties. The jet is a 50 '' wide, fanlike arrangement of line-emission knots extending up to 3' outside Cas A's main shell. Knot expansion velocities range between 7000 and 13,000 x (d/3 kpc) km s(-1) with the majority of radial velocities lying between -3000 to +1000 km s(-1), implying an orientation within 6 degrees of the plane of the sky and an opening angle of similar or equal to 25 degrees. The jet's optical emission consists of three knot types. Most show strong O, S, and Ar line emissions like other fast-moving knots (FMKs) in the remnant, but with comparatively weak [O III] line emission. A few jet FMKs show elongated morphologies suggestive of mass ablation due to Kelvin-Helmholtz instabilities along knot edges. Six high-velocity knots with strong [N II] emission (''fast-moving flocculi,'' or FMFs) are also found in the jet region. These FMFs lie along the southern edge of the jet and show a velocity range of 7000-9000 (d/3 kpc) km s(-1). In addition, about a dozen jet knots are found that exhibit both FMK- and FMF-type emissions, often with comparable line strengths. Such ''mixed emission knots'' or MEKs have an FMK-like velocity range of 8500-10,500 (d/3 kpc) km s(-1) and form a coherent and kinematically distinct structure. We suggest MEKs represent a debris hybrid caused by turbulent mixing of different progenitor layers. The Cas A jet may be the most visible example of a dozen or so ejecta plumes of core/mantle material in Cas A, possibly related to the high-speed expansion ''fingers'' seen in recent two-dimensional core-collapse models.