EVOLUTIONARY VERSUS DYNAMIC TIME SCALES FOR THE EVOLUTION OF THE CENTRAL STARS OF PLANETARY-NEBULAE

Investigated in this work is the disagreement between evolutionary and dynamical time scales for the evolution of the central stars of planetary nebulae (CSPNs) through the H-R diagram, using the results of high-resolution spectroscopic studies of CSPNs underway at the European Southern Observatory and at Palomar Observatory. These two studies combined have placed 23 CSPN in the distance-independent log g - log Teff diagram by comparing their photospheric absorption-line profiles observed at high signal-to-noise ratios with non-LTE (NLTE) model atmosphere line profiles. Results from the new Palomar 1.5 m echelle spectrograph confirm earlier published results from the ESO 3.6 m CASPEC echelle spectrograph for the three CSPNs the two samples have in common. Central star evolutionary ages deduced via comparisons with published evolutionary model calculations are poorly correlated with the dynamical expansion ages for the surrounding nebulae, prompting this investigation. Three possible reasons for this time scale 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; (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. While we cannot rule out mechanism (1) as a possible explanation, we find that the so-called born-again mechanism (2) is unlikely for the vast majority of CSPNs in our sample, owing to the very low ratio of dynamical age to interflash period. Our investigation of mechanism (3) demonstrates that the additional residual envelope masses required to reconcile the time scale disagreement are not unreasonable, and we favor this explanation because it is able to account for evolutionary ages both less than and greater than the corresponding dynamical ages. Estimates of residual envelope masses determined empirically as we have done may provide important clues toward an understanding of the AGB star to planetary nebula transition mechanism.