EVOLUTIONARY PATHS IN MULTIPLE-SHELL PLANETARY-NEBULAE

A homogeneous sample of 24 elliptical multiple-shell planetary nebulae (EM PNs) have been studied together with their central stars in order to disclose possible correlations between the stellar and nebular asp ects and to find the evolutionary paths yielding to these observed con figurations. We propose a finer classification for EM PNs, depending o n the appearance of the outer halo, based on detailed Ha surface brigh tness maps. We define detached-halo (DH), attached-halo (AH), and atta ched-halo/irregulars (AH/I) PNs. The evolutionary paths examined in co mparison to the observations were (1) multiple ejection at the thermal pulsating asymptotic giant branch phase (TP-AGB), including wind and superwind; (2) shell ejection during the postasymptotic giant branch ( post-AGB) cooling phase; and (3) asymmetric shell expansion. Our analy sis indicates that1. DH and AH PNs have different intrinsic nature, th ey show distinct galactic distributions, and they seem to originate th rough different evolutionary paths. DH PNs are round, while AH PNs oft en show marked ellipticity. 2. DH PNs might originate from multiple sh ell-ejection at the TP-AGB or from a ''born again'' episode, which is indicated by the spectral type of the central star. 3. Most AH PNs owe their shapes to dynamical processes due to winds interaction, mostly asymmetrical. 4. One AH/I and two AH PNs are probably projected bipola rs, as their chemistry suggests.