Super-AGB stars: A Nuclear Astrophysics Site for the I-Process
Elements heavier than iron are mainly formed by neutron-capture processes, though the respective astrophysical sites and conditions are still rather uncertain. Recent observations as well as new models and simulations have suggested that, in addition to the well-known slow- and rapid neutron capture processes, there may be an intermediate mode of neutron capture nucleosynthesis, the so-called i process. This process is defined by a neutron flux larger than those found in the well established slow neutron capture or s process, yet smaller than the extreme conditions of the rapid or r process. A possible signature of the i process  could be the simultaneous enhancement of Eu, usually considered an r-process element, and La, usually considered an s-process element, in some carbon enhanced metal-poor stars that have been classified as CEMP-r/s stars . Also pre-solar grains and post-AGB stars have been discussed earlier as a possible nucleosynthesis site for the i process, however, there are still discrepancies and open questions.
In a new study by Jones, Ritter, Herwig and collaborators , super-AGB stars are identified as another possible astrophysical site for the i process. In their new computational models of these very heavy AGB stars mixing at convective boundaries is taken into account according to a parameterized model. These new stellar evolution models suggest that proton-rich material could be convectively mixed into He-shell burning (Figure 1), leading to conditions suitable for the i process.
Interestingly, it could be shown that i-process conditions are more prominently found in models with lower metal content, indicating that the i process could have been more important in the early universe. In their paper the authors emphasize that 1D stellar evolution models can only identify possible sites for i-process nucleosynthesis, but that the Hingestion flashes are likely associated with substantial nuclear energy release, reaching maybe the level of the local binding energy of the Heburning shell. Such enormous energy input would launch an inherently 3D hydrodynamic global response that cannot be realistically described with 1D simulations . 3D stellar hydrodynamics simulations are needed to understand these nuclear astrophysics events fully and provide the appropriate context for further nuclear astrophysics investigations.
Researchers: S. Jones (UVic), C. Ritter (JINA-CEE & UVic), F. Herwig (JINA-CEE & UVic), C.Fryer (JINA-CEE & LANL), M. Pignatari (JINA -CEE & University of Hull), M.G. Bertolli (ORNL), B. Paxton (UC)