The Asymptotic Giant Branch (AGB) phase is the most productive evolutionary phase in terms of nucleosynthesis for low and intermediate mass stars. Nucleosythesis in these stars is aided by the mixing and heating triggered by recurrent He-shell flashes. Extreme nuclear energies (corresponding to 10^8L_sun) are generated during these thermonuclear flashes. This results in efficient shell convection with multiple implications for nucleosynthesis and further evolution. The evolution of AGB stars, including these He-flashes, has in the past been studied exclusively with one-dimensional stellar evolution codes that have to adopt simplifying assumptions on mixing, especially at convective boundaries. Here, we report on efforts to augment these studies with 2-d and 3-d hydrodynamic models of convective mixing in the AGB He-shell. We characterize the dominant morphology of He-shell flash convection. As opposed to the shallow surface convection in A-type stars studied by Freytag et al. (1996), coherently moving convective cells do not cross the convective boundary significantly. In other words, penetration is minimal for this convection zone. We find that convective motions induce a rich spectrum of internal gravity waves in the neighboring stable layers. Interactions of these (mainly horizontal) oscillations with the convective boundary, as well as oscillations with convective characteristics within the stable layers, do cause a finite amount of mixing across the convective boundary. Our preliminary analysis of this mixing is consistent with semi-analytical results obtained from observations and 1D-stellar evolution simulations.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 2006