6 Discussion

The Abell nebulae in this sample can be divided into two groups: truly spherical shells and bipolar nebulae viewed at different angles. Among the members of the first group are A 28, A 30, and A 33. A 13, although showing an elliptical shape, is not consistent with a spherical shell but rather represents a bipolar nebula viewed pole on. This planetary nebula would represent a "new ring'' object owing to its great similarity with NGC 6720. The remaining objects, A 21, A 24, and A 36 are bipolar nebulae similar to the general population of PNe. A systematic analysis of PNe images by Zhang & Kwok (1998) has found that most PNe have similar intrinsic (bipolar) structures and their diverse appearances are mainly a function of sky orientation and ionization effects. This interpretation is supported by a recent deep CCD imaging where faint bipolar nebulosities are revealed in PNe (e.g. SaWe3, Hua et al. 1998) previously not classified as bipolar. In this context, the existence of perfectly spherical nebulae is puzzling. In the interacting winds shaping model of Balick (1987), PNe evolve from spherical to elliptical to butterfly shapes as the fast wind exploits and amplifies the slight asymmetries in AGB envelopes. In this model, the different morphologies of PNe represent different stages of the evolutionary sequence. However, the most spherical PNe, the best examples being A 28, A 30, and A 33 in the present sample, are often the oldest and most evolved PNe. It is quite possible that some other mechanism, other than interacting winds shaping, is at work. By comparing the morphologies of young and evolved PNe, Aaquist & Kwok (1996) found that there is no significant difference in morphological class distribution between the two groups and suggested that the bipolar morphologies of PNe are created early in their evolution. From a sample of compact, young PNe observed by the HST, Sahai & Trauger (1998) suggested that the bipolar morphologies of PNe are shaped by non-isotropic, collimated, fast ejections that develop in the proto-planetary nebula phase. The detection of jet-like features in A 24 is interesting, but it is uncertain whether these features are related to the collimated outflows that create the lobes that lie (presumably) perpendicular to the plane of the sky. The nature of A 30 is still controversial. It has been argued that the spherical shell of A 30 came from a primary isotropic ejection that occurred some 10⁴ yr ago (the dynamical age derived from the shell expansion velocity) whereas the inner "ansae'' were formed more recently ($\sim$ 10³ yr) by an ejection of hydrogen-depleted material (Jacoby & Ford 1983). It has also been suggested that the second ejection is the result of "born-again'' evolution where the PN has suffered a late thermal pulse and returned to the AGB (Iben et al. 1983). However, the fact that A 30 has undergone a He ignition episode does not explain why the primarily shell should be spherical. For some reason, the fast collimated flow must not have developed in A 30. Whether this is related to the mass of the central star is a question that remains to be answered.