s-processing in AGB Stars Revisited. III. Neutron Captures from MHD Mixing at Different Metallicities and Observational Constraints

Abstract

We present post-process neutron-capture computations for Asymptotic Giant Branch (AGB) stars of 1.5–3 M ⊙ and metallicities −1.3 ≤ [Fe/H] ≤ 0.1. The reference stellar models are computed with the FRANEC code, using the Schwarzschild's criterion for convection; our motivations for this choice are outlined. We assume that MHD processes induce the penetration of protons below the convective boundary, when the Third Dredge Up occurs. There, the 13C n-source can subsequently operate, merging its effects with those of the 22Ne(α, n)25Mg reaction, activated at the temperature peaks characterizing AGB stages. This work has three main scopes. (i) We provide a grid of abundance yields, as produced through our MHD mixing scheme, which are uniformly sampled in mass and metallicity. From this, we deduce that the solar s-process distribution, as well as the abundances in recent stellar populations, can be accounted for, without the need of the extra primary-like contributions suggested in the past. (ii) We formulate analytic expressions for the mass of the 13C-pockets generated to allow easy verification of our findings. (iii) We compare our results with observations of evolved stars and with isotopic ratios in presolar SiC grains, also noticing how some flux tubes should survive turbulent disruption, carrying C-rich materials into the winds even when the envelope is O-rich. This wind phase is approximated through the G-component of AGB s-processing. We conclude that MHD-induced mixing is adequate to drive slow n-capture phenomena accounting for observations; our prescriptions should permit its inclusion into current stellar evolutionary codes.

Magnetic-buoyancy-induced mixing in AGB stars: Fluorine nucleosynthesis at different metallicities

“…r p can be identified from the critical toroidal B ϕ necessary for the onset of magnetic buoyancy instabilities (Vescovi et al 2020). k is the exponent quantifying the density decline, being ρ(r) ∝ r k , in the He-rich radiative layers below the convective envelope during a TDU, and it is typically lower than −3 (see also Busso et al 2021). The identification of the critical field necessary for the occurrence of instabilities by magnetic buoyancy allows identifying the corresponding radial position r p from which magnetic structures arise.…”

“…Post-process neutron-capture models for AGB stars in which the formation of the required 13 C reservoir is ascribed to mixing induced by magnetic buoyancy were shown to be able to account for the solar distributions of s-only isotopes (Trippella et al 2016), isotopic ratios of s-elements measured in presolar SiC grains (Palmerini et al 2018), and for a large part of the abundance observations in evolved low-mass stars (Busso et al 2021). The theory of mixing triggered by magnetic buoyancy developed by Nucci & Busso (2014) was used for this.…”

s-Processing in Asymptotic Giant Branch Stars in the Light of Revised Neutron-Capture Cross Sections

“…This causes the s-path to proceed towards 182 W and the 182 Hf production to be low. However, the radiogenic contribution of 182 Hf occurring at the end of the TP-AGB phase is important to explain the solar abundance of 182 W. Present AGB estimations for the s-process main component can account for 65-70% of solar 182 W [17,38], while its r-process contribution is well justified by the Galactic enrichment of r-process elements [39]. However, Lugaro et al [40] have pointed out that the present AGB contributions to 182 Hf and 182 W may have so far been underestimated.…”

“…More recently, the suggestion that stellar magnetic activity might be responsible for the formation of the 13 C pocket through mixing induced by magnetic buoyancy has been proposed [14,15]. Post-process calculations of such a 13 C reservoir have shown to be able to reproduce the distribution of s-elements in the solar main component [15], heavy-element isotopic compositions of presolar grains [16], and most of n-capture elements abundances observed in Ba-stars and post-AGB stars [17]. These studies have been confirmed by numerical simulations of the formation of a magnetically-buoyancy-induced 13 C pocket in a new series of FRUITY stellar evolutionary models [18].…”

The Gaia RVS benchmark stars

“…However, its luminosity is too low to be self-enriched in s−process elements because this should only happen when the star is in the phase of the thermal pulses (TP-AGB, see e.g. Busso et al 1999 andBusso et al 2021 andreferences therein). Furthermore, the overabundance of Eu precludes this interpretation because Eu is a pure r−process element.…”