The hydrodynamic environment for the s process in the He-shell flash of AGB stars

The He-shell flash convection in AGB stars is the site for the high-temperature component of the s-process in low- and intermediate mass giants, driven by the ²²Ne neutron source. During this phase several s-process branchings are activated, including some with time scales similar to the convective turn-over time scale. In addition, uncertainties regarding convective mixing at both the bottom and the top of the convective shell are preventing accurate predictions of s-process yields. The upper convection boundary plays a critical role during the Hingestion episode that may lead to neutron-bursts in the most metal-poor AGB stars. We address these problems through global 3-dimensional hydrodynamic simulations including the entire spherical He-shell flash convection zone (as oposed to the 3D box-in-a-star simulations). An important aspect of our current effort is to establish the feasibility of our appoach. We explain why we favour the explicit treatment over the anelastic approximation for this problem. The simulations presented in this paper use a Cartesian grid of 512³ cells and have been run on four 8-core workstations for four days to simulate ∼ 5000s, which corresponds to almost ten convective turn-over times. The convection layer extends radially at the simulated point in the flash evolution over 7H_p pressure scale-heights and exceeds the size of the underlying core. Convection is dominated by large convective cells that fill more than an entire octant. In order to better understand the conditions of the s-process branchings in this environment we have extracted particle tracers, and we discuss the thermodynamic trajectories along those paths.