i process review and convective-reactive burning in stars

Invited presentation given at the NIC XVI conference on Sep 21, 2021. Detailed nuclear astrophysics simulations have prompted the still relatively new realization that a distinct, new neutron-capture regime occurs in stars in addition to the well known slow (s) and rapid (r) processes can explain a wide range of observations, from abundance patterns in the CEMP-r/s stars, in certain post-AGB stars of solar metallicity, as well as in pre-solar grains. This new regime is characterized by a neutron density intermediate to the s- and r-process regime and is hence called the i process. Its nucleosynthetic path in the chart of isotopes runs parallel to the valley of stability, about 2-6 mass numbers into the neutron-rich side where radioactive beam facilities are needed to obtain the nuclear physics input data needed for modelling. Several astrophysical sites have been suggested, and some are now being explored using 3D hydrodynamic simulations with advanced post-professing techniques that reflect realistically the dynamic convective-reactive nature of the conditions in which the i process takes place. I will review i-process observations, modelling approaches and results, as well as the increasing number of nuclear physics impact studies that can guide future experiments. I will also discuss briefly a convective-reactive nucleoynthesis regime in massive stars when a convective O shell merges with the C shell. In this case odd-Z light elements as well as possibly p process elements are produced.