Rubidium, zirconium, and lithium production in intermediate-mass asymptotic giant branch stars

TL;DR

This study models the production of rubidium, zirconium, and lithium in intermediate-mass AGB stars, comparing predictions with observations to understand nucleosynthesis processes and their dependence on stellar parameters and metallicity.

Contribution

First simultaneous predictions of Rb, Zr, and Li abundances in intermediate-mass AGB star models, compared with observational constraints to refine understanding of nucleosynthesis.

Findings

Rb abundance increases with stellar mass, matching observations but not the highest Rb levels.

Including a partial mixing zone enhances Rb but overproduces Zr beyond observational limits.

Predicted Rb production increases with decreasing metallicity, aligning qualitatively with Magellanic Cloud observations.

Abstract

A recent survey of a large sample of Galactic intermediate-mass (>3 Msun) asymptotic giant branch (AGB) stars shows that they exhibit large overabundances of rubidium (Rb) up to 100--1000 times solar. These observations set constraints on our theoretical notion of the slow neutron capture process (s process) that occurs inside intermediate-mass AGB stars. Lithium (Li) abundances are also reported for these stars. In intermediate-mass AGB stars, Li can be produced by proton captures occuring at the base of the convective envelope. For this reason the observations of Rb, Zr, and Li set complementary constraints on different processes occurring in the same stars. We present predictions for the abundances of Rb, Zr, and Li as computed for the first time simultaneously in intermediate-mass AGB star models and compare them to the current observational constraints. We find that the Rb abundance increases with increasing stellar mass, as is inferred from observations but we are unable to match the highest observed [Rb/Fe] abundances. Inclusion of a partial mixing zone (PMZ) to activate the 13C(a,n)16O reaction as an additional neutron source yields significant enhancements in the Rb abundance. However this leads to Zr abundances that exceed the upper limits of the current observational constraints. If the third dredge-up (TDU) efficiency remains as high during the final stages of AGB evolution as during the earlier stages, we can match the lowest values of the observed Rb abundance range. We predict large variations in the Li abundance, which are observed. Finally, the predicted Rb production increases with decreasing metallicity, in qualitative agreement with observations of Magellanic Cloud AGB stars. However stellar models of Z=0.008 and Z=0.004 intermediate-mass AGB stars do not produce enough Rb to match the observed abundances.