Wind composition beyond the tip of the AGB and its relevance to stardust grains

After its H-rich envelope is reduced to a small mass, less than around 10−2 − 10−3 M , an asymptotic giant branch (AGB) star evolves to become a post-AGB star and then may become the central star of a planetary nebula. In these phases, the star evolves at a constant luminosity to hotter temperatures while the mass of the thin H-rich region decreases because of winds. We investigate the isotopic composition of this wind, which shows the signature of H burning. We compare our predictions to the composition measured in Group II stardust oxide and silicate grains to assess the hypothesis that some of this dust formed from material ejected by post-AGB stars and planetary nebula nuclei. We find that the composition of this H-rich region is very close to that of Group II grains, especially if some mixing of the different layers within the region is performed. The problem is that the total mass involved appears to be too small to reproduce the frequency of Group II grains. However, we cannot exclude this origin for Group II grains because their frequency would also be determined by the uncertain mechanism of grain formation in post-shock regions produced when fast winds from a planetary nebula central star collide with the material surrounding the star.