We performed 3-D simulations of proton-rich material entrainment into czw-rich He-shell flash convection and the subsequent H-ingestion flash that took place in the post-AGB star Sakurais object. Observations of the transient nature and anomalous abu
ndance features are available to validate our method and assumptions, with the aim to apply them to very low metallicity stars in the future. We include nuclear energy feedback from H burning and cover the full $4pi$ geometry of the shell. Runs on $768^3$ and $1536^3$ grids agree well with each other and have been followed for $1500mathrm{min}$ and $1200mathrm{min}$. After a $850mathrm{min}$ long quiescent entrainment phase the simulations enter into a global non-spherical oscillation that is launched and sustained by individual ignition events of H-rich fluid pockets. Fast circumferential flows collide at the antipode and cause the formation and localized ignition of the next H-overabundant pocket. The cycle repeats for more than a dozen times while its amplitude decreases. During the global oscillation the entrainment rate increases temporarily by a factor $approx 100$. Entrained entropy quenches convective motions in the upper layer until the burning of entrained H establishes a separate convection zone. The lower-resolution run hints at the possibility that another global oscillation, perhaps even more violent will follow. The location of the H-burning convection zone agrees with a 1-D model in which the mixing efficiency is calibrated to reproduce the light curve. The simulations have been performed at the NSF Blue Waters supercomputer at NCSA.
We present the first 3-dimensional, fully compressible gas-dynamics simulations in $4pi$ geometry of He-shell flash convection with proton-rich fuel entrainment at the upper boundary. This work is motivated by the insufficiently understood observed c
onsequences of the H-ingestion flash in post-AGB stars (Sakurais object) and metal-poor AGB stars. Our investigation is focused on the entrainment process at the top convection boundary and on the subsequent advection of H-rich material into deeper layers, and we therefore ignore the burning of the proton-rich fuel in this study. We find that, for our deep convection zone, coherent convective motions of near global scale appear to dominate the flow. At the top boundary convective shear flows are stable against Kelvin-Helmholtz instabilities. However, such shear instabilities are induced by the boundary-layer separation in large-scale, opposing flows. This links the global nature of thick shell convection with the entrainment process. We establish the quantitative dependence of the entrainment rate on grid resolution. With our numerical technique simulations with $1024^3$ cells or more are required to reach a numerical fidelity appropriate for this problem. However, only the result from the $1536^3$ simulation provides a clear indication that we approach convergence with regard to the entrainment rate. Our results demonstrate that our method, which is described in detail, can provide quantitative results related to entrainment and convective boundary mixing in deep stellar interior environments with veryvstiff convective boundaries. For the representative case we study in detail, we find an entrainment rate of $4.38 pm 1.48 times 10^{-13}M_odot mathrm{/s}$.
