Using a numerical method to treat nuclear burning and mixing simultaneously in stellar evolution calculations, which allows a reliable and robust modelling of very late thermal pulses, we have shown that the general surface abundance pattern observed in hydrogen-deficient post-AGB stars can be explained within the born-again scenario. Our new post-AGB sequence shows that due to the energy generation and convective mixing during a very late thermal pulse a born-again star forms which displays its previous intershell abundance at the surface.
We have based the calculation on an AGB model sequence computed with overshoot, which shows a high carbon and oxygen intershell abundance. Thus, the fact that the abundance pattern of our post-AGB model after the thermal pulse agrees with the observation of hydrogen-deficient post-AGB stars like PG 1159 and [WC]-CSPNe strongly supports the assumption of extra mixing beyond the convective boundary of the He-convection zone in AGB stars. We conclude that the very late thermal pulses can indeed be identified as one cause for the hydrogen-deficiency in post-AGB stars.
However, we note that not all H-deficient post-AGB stars are completely free of hydrogen (Leuenhagen and Hamann, 1998), as predicted by our model. Other possibilities than the born-again scenario to achieve H-deficiency might also exist (Tylenda, 1996; Waters et al., 1998). Whether post-AGB models which are not entirely hydrogen-free can be obtained within this scenario requires a study of the variation of the late thermal puls with the inter-pulse phase at which the star leaves the AGB (Iben, 1984), and possibly the consideration of other mixing processes, e.g. due to rotational effects (Langer et al., 1999), which has to be left to future investigations.
© European Southern Observatory (ESO) 1999
Online publication: August 25, 1999