Jets and the shaping of the giant bipolar envelope of the planetary nebulaKjPn 8

A hydrodynamic model involving cooling gas in the stagnation region of a co llimated outflow is proposed for the formation of the giant parsec-scale bi polar envelope that surrounds the planetary nebula KjPn 8. Analytical calcu lations and numerical simulations are presented to evaluate the model. The envelope is considered to consist mainly of environmental gas swept-up by s hocks driven by an episodic, collimated, bipolar outflow. In this model, wh ich we call the "free stagnation knot" mechanism, the swept-up ambient gas located in the stagnation region of the bow shock cools to produce a high-d ensity knot. This knot moves along with the bow shock. When the central out flow ceases, pressurization of the interior of the envelope stops and its e xpansion slows down. The stagnation knot, however, has sufficient momentum to propagate freely farther along the axis, producing a distinct nose at th e end of the lobe. The model is found to reproduce successfully the peculia r shape and global kinematics of the giant bipolar envelope of KjPn 8.