The mixing of internal gravity waves and lithium production in intermediate-mass asymptotic giant branch stars

School of Physical Science and Technology, Xinjiang University, Urumqi 830046, China

^★ Corresponding authors: guolianglv@sina.com; chunhuazhu@sina.cn

Received: 20 November 2024
Accepted: 19 June 2025

Abstract

Context. Intermediate-mass asymptotic giant branch (AGB) stars may have significant influence in the evolution of lithium (Li) in the Galaxy. During the AGB phase, stars eject surface material into interstellar matter (ISM) by stellar winds, and the Li content in their surfaces and winds decisively influences the AGB stars’ contribution of Li to ISM. Turbulent convection within stars - driven by internal gravity waves (IGWs) excited by convective motions - can transmit energy outward and induce mixing in non-convective regions, profoundly affecting the chemical composition of stellar surfaces and winds.

Aims. We aim to investigate the effects of IGWs on Li production of the AGB stars. We intend to demonstrate the validity of extra mixing triggered by IGWs in the radiative zone between the convective thermal pulse and the convective envelope of intermediatemass AGB stars and investigate its impact on Li production in these stars. In this paper, we use this model combined with initial mass functions to derive the total Li production from intermediate-mass AGB stars.

Methods. We used the Modules for Experiments in Stellar Astrophysics to construct stellar models from the zero-age main sequence to the end of the AGB. This simulation incorporated IGW-induced mixing coefficients and element diffusion effects. Grids were established to calculate the production of Li for AGB stars with different masses and metallicities. Subsequently, employing the method of population synthesis, we used linear interpolation in grids to estimate the contribution rate of Li production to the Galactic Li from a sample size of 10⁷ stars.

Results. Simulated results demonstrate that IGWs triggered during each Helium-shell flash induces extra mixing in non-convective regions. In our model, an IGW induces extra mixing that transports material from the radiative zone between the convective thermal pulse and the convective envelope to the convective envelope, allowing ⁷Be originally located below the convective envelope to be transported into the convective envelope, where ⁷Be decays into ⁷Li. The positive effects of IGW mixing on the Li yield diminish with increasing initial mass. For models with the same initial mass, the positive impact of IGW mixing increases with metallicity. In our calculations, most AGB stars with initial masses between 3.5 M_⊙ and 7.5 M_⊙ can produce a positive Li yields. Using the synthesispopulation method, we estimate that the total Li yields produced by AGB stars with IGW mixing are of about 15 M_⊙, which is twice that without IGW mixing. The contribution of these models to total galactic Li is around 10%. It means that an AGB star may be a non-negligible source for Li production.

Conclusions. Through this extra-mixing mechanism induced by IGWs, AGB stars can achieve a maximum A(Li) over 5, and intermediate-mass AGB stars significantly contribute to Li in the Galactic ISM. These findings underscore the crucial role of IGWs in stellar evolution, particularly in enhancing Li production.