Description
Most of the trans-iron elements in the Solar System were produced in the slow (s-) and rapid (r-) neutron-capture processes in stars and their explosions, which are both relatively well studied. On the other hand, abundances of heavy elements and their isotopes measured in some of the so-called carbon-enhanced metal-poor (CEMP) stars and presolar dust grains can be better explained as a result of i-process nucleosynthesis that occurred at neutron densities intermediate between those
characteristic for the s- and r-processes. The most likely site of the i process is a helium convective zone that entrains hydrogen from an adjacent H-rich envelope. This may happen in rapidly-accreting white dwarfs (RAWDs) in close binary systems, like those considered to lead to Supernova Ia explosions in the single-degenerate channel, or in asymptotic giant branch (AGB) stars at low metallicities.
I will present and compare results of numerical simulations of i-process nucleosynthesis in a simple
one-zone model for a range of constant neutron densities as well as in more realistic multi-zone models of RAWDs and AGB stars. The abundances of heavy elements and their isotopes obtained in those models will be compared with elemental and isotopic abundance ratios measured in CEMP stars and presolar dust grains. On the chart of nuclides the i process path extends from 3 to 8 isotopes beyond the valley of stability, for most of which only theoretical neutron-capture reaction rates computed using the Hauser-Feshbach method are available. Uncertainties of these rates are likely to be responsible for some discrepancies between the predicted and observed abundances. Which of these uncertainties may have the strongest impact on the predicted abundances of selected elements and isotopes can be revealed in Monte Carlo (MC) simulations, in which all rates are varied within their limits estimated from Hauser-Feshbach computations, followed by a statistical analysis of their results. I will present and compare results of such MC simulations and their analysis obtained for the one- and multi-zone models of i-process nucleosynthesis, in particular an updated list of reactions whose rates need to be constrained experimentally and their uncertainties reduced for the predicted abundances to better agree with the observed ones.
Finally, I will advertise the CaNPAN i-process computational tools that can be used to conduct reaction rate uncertainty studies for one-zone nucleosynthesis models.
