The mixing of internal gravity waves and lithium production in intermediate-mass AGB stars

TL;DR

This study models how internal gravity waves induce extra mixing in intermediate-mass AGB stars, significantly boosting lithium production and contributing to Galactic lithium enrichment.

Contribution

It introduces a detailed stellar model incorporating IGW-induced mixing, revealing its impact on Li yields in AGB stars, a novel aspect in stellar evolution modeling.

Findings

IGW triggers extra-mixing during He-shell flashes.

Most AGB stars (3.5-7.5 solar masses) produce positive Li yields.

Total Li yield with IGW mixing is about twice that without, contributing ~10% to Galactic Li.

Abstract

Context: Intermediate-mass asymptotic giant branch (AGB) stars influence Galactic lithium evolution by ejecting surface material (including Li) via stellar winds. Internal gravity waves (IGW), excited by convective motions, drive turbulent mixing in non-convective regions, altering stellar surface and wind chemistry. Aims: Investigate IGW-induced extra-mixing in the radiative zone between thermal pulses and convective envelopes of AGB stars and its impact on Li production. Derive the total Li contribution from intermediate-mass AGB stars using stellar models and initial mass functions. Methods: Construct stellar models (from zero-age main sequence to AGB end) with MESA, incorporating IGW-induced mixing and element diffusion. Calculate Li yields for stars of varying masses and metallicities using grids and population synthesis. Results: IGW triggers extra-mixing during He-shell flashes, transporting Be-7 from the radiative zone to the convective envelope, where it decays into Li-7. The positive effect of IGW on Li yield decreases with initial stellar mass but increases with metallicity. Most AGB stars (3.5-7.5 solar masses) produce positive Li yields. The total Li yield with IGW mixing (approximately 15 solar masses) is twice that without, contributing about 10 percent to Galactic Li. Conclusions: Through this extra-mixing mechanism induced by IGW, AGB stars can achieve a maximum A(Li) exceeding 5 and intermediate-mass AGB stars significantly contribute to Li in the Galactic ISM. These findings underscore the crucial role of IGW in stellar evolution, particularly in enhancing Li production.