GLOBAL NON-SPHERICAL OSCILLATIONS IN THREE-DIMENSIONAL 4π SIMULATIONS OF THE H-Ingestion FLASH

Abstract

We performed 3-D simulations of proton-rich material entrainment into 12 C-rich He-shell flash convection and the subsequent H-ingestion flash that took place in the post-AGB star Sakurai's object. Observations of the transient nature and anomalous abundance 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 4π geometry of the shell. Runs on 768 3 and 1536 3 grids agree well with each other and have been followed for 1500min and 1200min. After a 850min 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 ≈ 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.

Post-AGB stars in the Magellanic Clouds and neutron-capture processes in AGB stars

“…There are as yet no detailed models of this process for low-metallicity AGB stars, although PIE events have been identified as a a possible physical site (the i-process, Cowan & Rose 1977;Herwig et al 2011). Overshoot leading to PIEs is uncertain, but known to be favoured in AGB stars of mass ∼1 M and [Fe/H] < 2 (Fujimoto et al 1990;Campbell & Lattanzio 2008;Cristallo et al 2009;Lugaro et al 2012) with recent 3D models supporting these results (Stancliffe et al 2011;Herwig et al 2014;Woodward et al 2015). We stress that one-dimensional hydrostatic models of PIE events with artificial proton distributions like those presented here are not expected to describe the ingestion and the mixing correctly, and this may have a strong effect on the resulting abundance patterns.…”

i-process Nucleosynthesis and Mass Retention Efficiency in He-shell Flash Evolution of Rapidly Accreting White Dwarfs

“…If it is confirmed in 3D simulations that no Global Oscillation of Shell H-ingestion (GOSH, Herwig et al 2014) is found in this situation, 1D spherically symmetric simulations of the i-process nucleosynthesis in RAWDs are probably more realistic compared to the case of post-AGB VLTP models and low-Z AGB models for which 3D stellar hydrodynamics simulations have yet to determine the detailed outcome of the GOSH. At a later time (t = 174.8yr) the H-ingestion rate does increase for a brief period when the convection zone is indeed split.…”

Pop III i-process nucleosynthesis and the elemental abundances of SMSS J0313−6708 and the most iron-poor stars

“…1), and the burning of H creates a split in the He-shell, similar to Herwig et al (2011). 3D simulations are only starting to investigate this process with the necessary numerical effort (Herwig et al 2014), but already show that violent, global instabilities are possible. The 3D behaviour is expected to be fundamentally different compared to what is seen in 1D stellar evolution models.…”