The Shape of Cas A

On the basis of optical, IR, and X-ray studies of Cas A, we propose a geometry for the remnant based on a "jet-induced" scenario with significant systematic departures from axial symmetry. In this model, the main jet axis is oriented in the direction of strong blueshifted motion at an angle of 110°-120° east of north and about 40°-50° to the east of the line of sight. Normal to this axis would be an expanding torus as predicted by jet-induced models. In the proposed geometry, iron-peak elements in the main jetlike flow could appear "beyond" the portions of the remnant rich in silicon by projection effects, not the effect of mixing. In the context of the proposed geometry, the displacement of the compact object from the kinematic center of the remnant at a position angle of ~169° can be accommodated if the motion of the compact object is near to, but slightly off from, the direction of the main "jet" axis by of order 30°. In this model, the classical NE "jet," the SW "counterjet," and other protrusions, particularly the "hole" in the north, are nonaxisymmetric flows approximately in the equatorial plane, e.g., out through the perimeter of the expanding torus, rather than being associated with the main jet. We explore the spokelike flow in the equatorial plane in terms of Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholz instabilities and illustrate these instabilities with a jet-induced simulation.