Published online by Cambridge University Press: 12 April 2016
After a brief commentary on the place of “peculiar red giants” in the overall scheme of stellar evolution, an outline is given of the various possibilities for post asymptotic giant branch (AGB) evolution. The behavior of a post-AGB model star is crucially dependent on where in a thermal pulse cycle the mass of the hydrogen-rich envelope is reduced to such an extent that departure from the AGB must follow on a thermal time scale. If departure from the AGB occurs while the model is still burning hydrogen, post-AGB behavior depends on the mass of the helium buffer zone (- zone containing predominantly helium which has been processed through the hydrogen-burning shell following the last thermal pulse on the AGB). If departure occurs at an arbitrary time during the hydrogen-burning phase, then: (1) in - 25% of all cases, the post-AGB model will experience a final helium shell flash, and, in consequence of additional mass loss, may become a non-DA white dwarf; (2) in - 60% of all cases, the model will cease burning hydrogen when the mass in its hydrogen-rich envelope is reduced to ∼ 10-4Mʘ and will evolve into a DA white dwarf; and (3) in - 15% of all cases, the model will experience a final hydrogen shell flash, but the outcome with regard to spectroscopic type is unclear. If departure from the AGB occurs while the model is burning helium, the result is either the same as in option (3) just described, or mass loss during the post-AGB helium-burning phase may turn the star into a non-DA white dwarf.