Impact of 3D macro physics and nuclear physics on the s-process during He-flashes in Asymptotic Giant Branch stars

The He He -intershell in Asymptotic Giant Branch stars is a main site for the s -process, but 1D stellar models rely on the mixing length theory, which does not match the radial velocity profiles of 3D hydrodynamic simulations of He He -flashes. We investigate how 3D macro physics-informed mixing impacts the nucleosynthesis of s -nuclei. We post-process the pulse-driven convective zone of the MZAMS=3\; M_ M ZAMS = 3 M ⊙ , $$Z=0.01$$ Z = 0.01 model from the NuGrid stellar data set. Applying a downturn to velocities at the boundary and increasing velocities across the shell as obtained in previous results, we find that, within a single He He -flash, adopting 3D-inspired mixing profiles leads to only minor variations in the production of s s -process isotopes, typically at the level of fractions of a δ δ point in zirconium and barium isotopic ratios. In contrast, nuclear physics uncertainties, particularly those associated with the ²²Ne(α ,n) ²⁵Mg 22 Ne ( α , n ) 25 Mg neutron source, produce significantly larger effects. Our results nevertheless suggest that the decreased mixing efficiency at the bottom of the pulse-driven convection zone predicted by hydrodynamic simulations may lead to structural changes if implemented self-consistently in full stellar evolution models from the onset of the AGB phase, potentially affecting the efficiency of convection-induced mixing beneath the 1D Schwarzschild boundary of the He He intershell thermal pulses boundary. However, establishing robust parametrisations of such effects will require 3D hydrodynamic simulations of the He He -flash that include sufficiently deep stable layers below the convective region.