The i-process yields of rapidly accreting white dwarfs from multicycle He-shell flash stellar evolution models with mixing parametrizations from 3D hydrodynamics simulations

We have modelled the multicycle evolution of rapidly accreting CO white dwarfs (RAWDs) with stable 11 burning intermittent with strong He-shell flashes on their surfaces for 0.7 <= M-RAWD/M-circle dot <= 0.75 and Weil ranging from 0 to -2.6. We have also computed the i-process nucleosynthesis yields for these models. The i process occurs when convection driven by the He-shell flash ingests protons from the accreted II-rich surface layer, which results in maximum neutron densities N-n,N- max R approximate to 10(13)-10(15) cm(-3). The II-ingestion rate and the convective boundary mixing (CBM) parameter f(top), adopted in the one-dimensional nucleosynthesis and stellar evolution models are constrained through three-dimensional (3D) hydrodynamic simulations. The mass ingestion rate and, for the first time, the scaling laws for the CBM parameter f(top) have been determined from 31) hydrodynamic simulations. We confirm our previous result that the high-metallicity RAWDs have a low mass retention efficiency (eta less than or similar to 10 per cent). A new result is that RAWDs with [Fe/H] less than or similar to -2 have eta greater than or similar to 20 per cent; therefore, their masses may reach the Chandrasekhar limit and they may eventually explode as SNcIa. This result and the good fits of the i-process yields from the metal-poor RAWDs to the observed chemical composition of the CEMP-r/s stars suggest that some of the present-day CEMP-r/s stars could be former distant members of triple systems, orbiting close binary systems with RAWDs that may have later exploded as SNcIa.