SEARCH FOR IONIZED CORES IN PROTO PLANETARY-NEBULAE, AND THE ASYMPTOTIC GIANT BRANCH TO PLANETARY-NEBULA TRANSITION

In order to compare the evolutionary time of planetary nebula nuclei ( PNNs) to the kinematical age of the planetary nebula (PN) one has to s pecify the AGB to PN transition time, i.e., the time taken by the cent ral star to become hot enough for hydrogen ionization (approximately 3 0,000 K). We have shown that, whatever the physical process driving th e transition, the transition time depends on the residual envelope mas s after the cessation of the AGB superwind, i.e., on a quantity that c urrent theory can hardly predict at all. Given this serious limitation , we have envisaged an empirical approach which can set limits on the transition time, and possibly determine its average value. To this end we have preliminarily evaluated the time taken by the circumstellar d ust shells to become optically thin at visible and near-IR wavelengths , following the termination of the superwind envelope ejection. Typica l values of these thinning times turns out to be approximately 1000 an d approximately 100 yr, respectively at Halpha and Bralpha wavelengths . We then proceeded to observe a sample of candidate transition object s (mostly non-variable OH/IR sources), to check whether some exhibits Bralpha emission signaling the presence of an ionized nebular core, an d therefore setting a constraint on the relative duration of the trans ition and thinning times. In a sample of 21 objects we detected Bralph a emission in only one case, while Halpha is also detected, albeit ver y weak because of circumstellar extinction. If the typical transition time had been shorter than the thinning time we should have found Bral pha emission in a fair fraction of our targets. These findings imply f or most post-AGB stars transition times in excess of approximately 100 0 yr, and therefore the transition time may be comparable to the nebul ar age in a fair fraction of known planetary nebulae.