Masses and Evolution of Central Stars of Planetary Nebulae

Abstract

The evolutionary paths of nuclei of planetary nebulae (NPN) in the HR-diagram have been subject to considerable controversy during the past. The empirically derived Harman-Seaton sequence was generally interpreted as a single evolutionary track, leading into the white dwarf region with a characteristic upturn from about 103 to 104 L⊙ and being traversed within the lifetime of the PN, i.e. about 30000 years. Theoretical calculations, however, were unable to reproduce such a track, showing instead a nearly horizontal evolution from the red giant into the white dwarf region at a luminosity level which was dictated by the core mass-lumino- sity relation at the asymptotic giant branch (Paczynski,1971, Härm and Schwarzschild,1975). The observed loci of the NPN could thus be interpreted not as a single track but as a superposition of tracks for NPN of different masses (for a discussion see O’Dell, 1974). The empirical situation changed after Pottasch et al.(1978) derived lower effective temperatures for many of the NPN at the top of the Harman-Seaton sequence and also obtained lower luminosities, showing no indication of an upturn or a sequence. Weidemann (1977a) tried to interpret the new situation by pointing out that the narrow mass range derived for white dwarfs, around 0.6 µ⊙, if assumed to hold also for NPN, would predict a nearly single evolutionary track for NPN again which would show no upturn but should run along the luminosity level given by the core mass - luminosity relation for 0.6 µ⊙, before descending into the white dwarf region. Renzini (1979),however, on reconsidering the Paczynski tracks, with time intervals for the visibility of PN marked, proposed another interpretation, according to which different parts of the observed NPN sequence are populated by stars with different masses (I), the brightest NPN have rather low mass,0.5-0.7. µ⊙ whereas the faint NPN have higher mass, 0.8 - 1.2µ⊙ or more (II), bright NPN, coming from low mass stars, should have different kinematical properties with respect to faint NPN (with higher progenitor masses) (III), the nebular masses should be significantly higher for faint NPN (IV), the chemical composition should be a function of the location on the sequence (V), and, contrary to Weidemann (1977a), there should be a luminosity upturn in the low temperature branch (VI).KeywordsExpansion VelocityCentral StarPlanetary NebulaAsymptotic Giant BranchLuminosity RelationThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.