CEMP-r/s stars and other observations of the i process - models, simulations, nucleosynthesis and the impact of nuclear physics uncertainties

Falk Herwig, University of Victoria

Abstract: The origin of CEMP stars with both s- and r-process abundance signature has been a puzzle since they were identified almost 20 years ago. A number of scenarios have been proposed to explain these peculiar metal-poor stars, all of which include either ad-hoc assumptions and/or make abundance predictions that are not representing the observed abundance patterns very well. In recent years it has become clear that a neutron-capture process with neutron densities intermediate between those of s- and r-process can account for many details of the observed abundance distribution in many CEMP-r/s stars. They key elements are in the Ba to Ir region between the second and third peak, and a tell-tale signature is the high ratio of elements Er - Hf compared to Ir and Os, the latter being bona fide r-process elements. These early results were based on simplistic one-zone nucleosynthesis simulations, and for several years it was unclear as to what the actual astrophysical site of the i process for CEMP-r/s stars is. We have now discovered that low-metallicity rapidly accreting  white dwarfs are an extremely promising candidate site. In this talk I will explain why this somewhat exotic site is such a good candidate, and why predictions from this site are much less uncertain than from any other i-process site considered previously. This will include an account of our recent hydrodynamic simulations of various i-process sites, including our new 3D1D hyrdo-nucleosynthesis approach. I will also summarize the results of three i-process nuclear physics impact studies that we have conducted, spanning combined the mass range all the way from just above Fe to Hf and show that present uncertainties in in nuclear data of (n,g) captures of species 2-5 masses away from the valley of stability can cause uncertainties in predicted abundance ratios of up to 1dex.