STELLAR EVOLUTION OF LOW AND INTERMEDIATE-MASS STARS .3. AN APPLICATION OF EVOLUTIONARY POST-AGB MODELS - THE VARIABLE CENTRAL STAR FG SAGITTAE

Based on a set of evolutionary calculations for thermally pulsating po st-AGE models we introduce a robust method to measure FG Sge's mass wh ich was found to be 0.61 M.. The corresponding evolutionary timescale is consistent with the expansion age of the planetary nebula. Assuming that FG Sge's surface was already enriched with heavy elements on the AGE we propose that during the central-star evolution these elements were removed from the superficial radiative layers due to, e.g., dust/ gas separation, leaving the deeper convective layers unchanged. During the flash the outward moving envelope convection mixed the stored hea vy elements back to the surface.