Observational Constraints to the Theory of Planetary Nebulae Evolution

Abstract

Two main observational points regarding Planetary Nebulae (PNe) are discussed in the framework of our theoretical knowledge of stellar evolution, namely:

• i) the expansion age of young PNe is much shorter than the age which the PN nuclei (PNN) would have if they were in the stage of shell hydrogen burning, and compare much better with the thermal timescales of the PNN. This indicates that the expulsion of the PN leaves on the PNN a very small or even negligible hydrogen envelope, as it is expected on the basis of the few existing hydrodynamic models.

• ii) the morphological subdivision of PNe into two main classes (B and C) appears to correspond also to a subdivision in terms of physical parameters both of the nebula and of the central star, indicating that asymmetric “bipolar” nebulae, at least statistically descend from more massive progenitors with respect to symmetric PNe. It is discussed whether the asymmetries can be imputed to common envelope evolution in binaries, which would give origin to the B nebulae, or to the different core rotation of more massive progenitors. In any case, the two classes B and C seem to correspond to a subdivision between progenitors which suffer or do not suffer the helium flash.