MORPHOLOGICAL POPULATIONS OF PLANETARY-NEBULAE - WHICH PROGENITORS .1. COMPARATIVE PROPERTIES OF BIPOLAR NEBULAE

We have investigated morphological vs. physical properties of planetary nebulae (PNe). The sample consists of about 400 objects with high quality optical images. An extensive list of 43 bipolar (butterfly) nebulae has been built by adding to our data several objects found in the literature. Particular emphasis has been given to the comparison between the elliptical and bipolar nebulae. We find that the latter class has: a) a scale height on the Galactic plane of 130 pc compared to the value of 320 pc for the elliptical objects and of 260 pc for the global sample of Galactic disk PNe; b) smaller deviations than the other morphological types from pure circular Galactic rotation; c) the hottest central stars among PNe; d) chemical overabundaces of helium, nitrogen and neon; e) outflow velocities up to one order of magnitude larger than the typical expansion velocities of PNe; f) giant dimensions. We also suggest empirical evolutionary tracks for PNe in the two-colour IRAS diagram. The above properties indicate that bipolar PNe are produced by more massive progenitors than the other morphological classes. We conservatively adopt a lower limit of 1.5 M(circle dot) for the progenitors of bipolar PNe and solar values (M less than or similar to 1.1 M(circle dot)) for the ellipticals. The present results can be used to constrain the discussion of the AGE mass loss geometry and the formation of the different morphological classes of PNe. Models considering single or interacting binary stars are reviewed. The peculiar nature of M 2-9 is discussed separately.