Thermal pulses and the formation of planetary nebula shells.

Over the past decade a comprehensive, semiquantitative theoretical scenario for the final evolutionary stages of low and intermediate mass stars has been progressively elaborated and refined. It concerns the envelope ejection terminating the asymptotic giant branch (AGB) phase, the AGB to planetary nebula (PN) transition, the fading and possible rejuvenation of PN nuclei, the formation processes of hydrogen-deficient stars, and the final production of white dwarfs of the DA and non DA-varieties. In the present communication an attempt is presented to further enrich this scenario, by explicitly considering some aspects which heretofore have remained insufficiently explored.