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A hydrodynamic model involving cooling gas in the stagnation region of acollimated outflow is proposed for the formation of the giant parsec-scalebipolar envelope that surrounds the planetary nebula KjPn 8. Analyticalcalculations and numerical simulations are presented to evaluate the model. Theenvelope is considered to consist mainly of environmental gas swept-up byshocks driven by an episodic, collimated, bipolar outflow. In this model, whichwe call the ``free stagnation knot'' mechanism, the swept-up ambient gaslocated in the stagnation region of the bow-shock cools to produce a highdensity knot. This knot moves along with the bow-shock. When the centraloutflow ceases, pressurization of the interior of the envelope stops and itsexpansion slows down. The stagnation knot, however, has sufficient momentum topropagate freely further along the axis, producing a distinct nose at the endof the lobe. The model is found to successfully reproduce the peculiar shapeand global kinematics of the giant bipolar envelope of KjPn 8.Comment: 20 pages + 8 figures (in 1 tar-file 0.67 Mb

Jets and the Shaping of the Giant Bipolar Envelope of the Planetary Nebula KjPn 8