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

TL;DR

This study models the evolution and nucleosynthesis yields of rapidly-accreting white dwarfs, revealing their potential to reach supernova conditions and explaining certain stellar chemical signatures through detailed 3D hydrodynamic simulations.

Contribution

It introduces new scaling laws for convective boundary mixing parameters derived from 3D simulations and links white dwarf evolution to observed stellar abundances.

Findings

High-metallicity RAWDs have low mass retention efficiency (<10%).

Low-metallicity RAWDs ([Fe/H]<-2) can retain >20% mass, possibly leading to supernova explosions.

i-process nucleosynthesis yields match observed chemical patterns in CEMP-r/s stars.

Abstract

We have modelled the multicycle evolution of rapidly-accreting CO white dwarfs (RAWDs) with stable H burning intermittent with strong He-shell flashes on their surfaces for $0.7\leq M_\mathrm{RAWD}/M_\odot\leq 0.75$ and [Fe/H] 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 H-rich surface layer, which results in maximum neutron densities $N_\mathrm{n,max}\approx 10^{13}$-$10^{15}\ \mathrm{cm}^{-3}$. The H-ingestion rate and the convective boundary mixing (CBM) parameter $f_\mathrm{top}$ adopted in the one-dimensional nucleosynthesis and stellar evolution models are constrained through 3D hydrodynamic simulations. The mass ingestion rate and, for the first time, the scaling laws for the CBM parameter $f_\mathrm{top}$ have been determined from 3D hydrodynamic simulations. We confirm our previous result that the high-metallicity RAWDs have a low mass retention efficiency ($\eta < 10\%$). A new result is that RAWDs with [Fe/H]$< -2$ have $\eta > 20\%$, therefore their masses may reach the Chandrasekhar limit and they may eventually explode as SNeIa. 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 SNeIa.