Evolutionary versus dynamical time scales for the evolution of the central stars of planetary nebulae

The disagreement between evolutionary and dynamical time scales for the evolution of the central stars of planetary nebulae (CSPN) through the H-R diagram is investigated. Three possible reasons for the disagreement are examined: (1) the nebulae could have experienced a phase of rapid photoionization of material ejected previously while the stars were still on the AGB; (2) the central stars could have undergone a late helium shell flash and returned to the AGB; and (3) the AGB-CSPN evolutionary transition times could have been increased by small additional amounts of residual envelope material remaining after the superwind mass-loss phase. Mechanism (3) is favored here because it can account for evolutionary ages both less than and greater than the corresponding dynamical ages. 45 refs.