Distribution and Evolution of the Li Abundance in Red Clump Stars Can Be Explained by Internal Gravity Waves

Abstract

The study of Li phenomena in red clump (RC) stars can give us a deeper understanding of the structure and evolution of stars. Chanamé et al. explained the RC Li abundance distributions naturally using only standard post-main-sequence (MS) Li evolution models when the distribution of progenitor masses and the depletion of Li during the MS observed in MS stars were considered, thus neither an extra Li depletion nor Li creation mechanism is required. Nevertheless, it is interesting to consider the effects of mixing caused by some extra mechanisms. By constructing different models, we find that the mixing caused by internal gravity waves can explain the observed Li abundances of RC stars with low-mass progenitors. To explain this, we rely on the extra mixing induced by internal gravity waves that are excited at the bottom of the convective envelope during the red giant branch (RGB) stage. During the RGB stage, introducing internal gravity waves can improve the diffusion coefficient and strengthen the mixing effect. The effective enrichment of Li occurs during the late RGB stage and requires the diffusion coefficient of the H-burning shell to reach ∼108 cm2 s−1. Our models predict that the Li abundance decreases from ∼1.5 to ∼0.0 dex at the end of the core He-burning stage, thereby revealing ∼99% of the observed Li abundance distribution. Thermohaline mixing regulates the Li abundance of RGB stars, which combined with internal gravity waves can explain the Li abundances of most giants.