Precessing jets and point-symmetric nebulae

We present a model for the formation of point-symmetric nebulae that relies on the existence of a precessing jet interacting with the interstellar medium (ISM). Using three-dimensional numerical simulations, we investigate the basic gasdynamics inherent to the model. Through “synthetic observations” of our simulations we show that episodic precessing jets can reproduce the gross morphological structure of point-symmetric nebulae, i.e., a string of discrete clumps in an S-shaped intensity distribution. We also find that the bow shocks of the individual jet segments can merge into a single shock structure that envelops the entire complex of segments. The development of this enveloping shock allows the model to embrace nebulae consisting of discrete point-symmetric clumps as well as those bipolar objects that show nonuniform brightness distributions on their opposing lobes that are point symmetric through the nucleus. By demonstrating that these bipolar planetary nebulae can form from the same mechanism which produces the discrete point-symmetric nebulae, we can include them in the category of point-symmetric objects, thereby increasing their fractional occurrence in planetary nebulae by 75%.