Gas dynamics in planetary nebulae: From macro-structures to FLIERs

Purpose of this paper is to clarify how Planetary Nebulae (PNe) are very in teresting laboratories to study cosmic gas dynamics. I first recall the his tory of PNe which are generated from low and intermediate mass stars throug h successive mass loss processes starting in the Reg Giant phase of evoluti on and continuing also after the termination of the pulsed AGB phase, where most of the nebular mass is believed to be ejected. The correponding stell ar winds are the ingredients of the nebula. Their initial properties and su bsequent mutual interactions, under the action of the evolving stellar radi ation field, are responsible for the properties of the nebula. The observed structures of PNe are considered in detail. Larger scale macroscopic struc tures (MACS) are examined separately from quite smaller scale microscopic s tructures (MICS). The formation of MACS, at least in cases of round to mode rately elliptical PNe, is shown to be reasonably well understood in terms o f existing hydrodynamical models. Considering the kinematical behaviour, MI CS can be separated into FLIERs (Fast Low Ionization Emitting Regions) and SLOWERs (slowly moving). Attention is focussed on FLIERs and on the propose d mechanisms to interpret them. Recent observations with the Hubble Space T elescope have provided us with a wealth of detailed (subarcsec) information on the nebular structures. The inner structure of FLIERs is here illustrat ed to consist of substructures of various shapes with an high degree of ind ividually from object to object, also within the same PN. These new data ca ll for deeper thoretical efforts to solve the problems of cosmic gas dynami cs, posed by their observed properties. An ample account is given of the mo st relevant original works, in an effort to allow the non specialist reader to quickly become acquainted with the status of art in the various aspects of the subject.