Wind beyond the tip of the AGB and its relevance to stardust grain data

An asymptotic giant branch (AGB) star evolves to the post-AGB phase and then may become the core of a planetary nebula. The star evolves at a constant luminosity to hotter temperatures while the mass of the thin H-rich region decreases because of winds. Stardust oxide and silicate grains are formed around AGB stars and recovered from meteorites. The origin of the "Group II grains" is currently explained by invoking the occurence of some kind of extra-mixing process in AGB stars. We propose an alternative explanation: these grains may have originated from the winds of post-AGB stars and planetary nebula cores. We predict that this wind shows the signature of Hburning. We compare our predictions from stellar models to the compositions observed in Group II stardust oxide and silicate grains to assess this hypothesis. We find that the O- and Al-isotopic composition of the winds is very close to that of Group II grains, especially if some mixing of the ejected material is performed. However, the Mg-isotopic composition of the winds does not provide as close a match to the Group II grains.