Microstructures of Planetary Nebulae with Large Telescopes

Planetary nebulae (PNe) are known to possess a variety of small-scale structures that are usually in a lower ionization state than the main body of the nebulae. The morphological and kinematic properties of these low-ionization structures (LISs) vary from type to type in the sense that LISs can appear in the form of pairs of knots, filaments, jets, and isolated features moving with velocities that either do not differ substantially from that of the ambient nebula, or instead move supersonically through the environment. The high-velocity jets and pairs of knots, also known as FLIERs, are likely to be shock-excited. So far, most of the FLIERs analyzed with ground-based small and medium telescopes, as well as with the HST, do not show the expected shock-excited features —either the bow-shock geometry or the shock excited emission lines. In this talk we discuss the crucial problem of the excitation mechanisms of FLIERs —through the comparison of jets and knots of NGC 7009 and K 4-47— and what might be the contribution of large telescopes. MACROAND MICROSTRUCTURES OF PNE The main, macro, structures of PNe are the elliptical, bipolar, or point-symmetric rims and shells, as well the haloes that dominate the optical [O III] and Hα line emission of planetary nebulae (see top panel of Figure 1). Notwithstanding important revisions, the mechanism responsable for the rim formation has been known for almost 30 years as the interacting stellar wind, ISW, models (Kwok, Purton & FitzGerald 1978). Now we believe that, in addition to the interplay between the slow AGB and the fast post-AGB winds, magnetic fields and rotation —probably due to a disk within a binary system— should also play a role in the formation of the main structures of the PNe (for a review, see Balick & Frank 2002). The formation of the shells, external to the rims, has been ascribed to the action of the photoionization front on the AGB matter not yet reached by the shock produced by the fast wind (e.g., Schönberner 2002). The outer halo in its turn is interpreted by Corradi et al. (2003) as being composed by matter ejected during the AGB phase, its outer edge marking the signature of the last AGB thermal pulse. The microstructures of PNe, on the other hand, are far less understood theoretically. That they are much more prominent in the lower ionization optical emission lines ([N II], [S II], [O II]), appear on smaller scales than the main structures, and are usually different of the main bodies in terms of morphology and kinematics, gives us some guidelines in attempting to unveil their origin. A number of papers have recently been published on the subject, in which some aspects of their origin are treated. From observations, we know that: LISs appear as pairs of jets, knots, filaments, and jetlike features, or as isolated systems (bottom panel of Figure 1); LISs sometimes expand with the rim, shells, or FIGURE 1. Morphological classes of PNe. Top: The main components. Bottom: The microstructures,