Li-rich and super Li-rich giants produced by element diffusion

Abstract

Context. About 0.2−2% of giant stars are Li rich, that is to say their lithium abundance (A(Li)) is higher than 1.5 dex. Nearly 6% of these Li-rich giant stars are super Li rich, with an A(Li) exceeding 3.2 dex. Meanwhile, the formation mechanism of these Li-rich and super Li-rich giants is still under debate. Aims. Considering the compact He core of red giants, attention is paid to the effect of element diffusion on A(Li). In particular, when the He-core flash occurs, element diffusion makes the thermohaline mixing zone extend inward and connect to the inner convection region of stars. Then, a large amount of 7Be produced by the He flash can be transferred to the stellar surface, finally turning into 7Li. Thus, the goal of this work is to propose the mechanism of A(Li) enrichment and achieve consistency between the theoretical and observation data. Methods. Using the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code, we simulated the evolution of low-mass stars, considering the effects of element diffusion on the Li abundances. The timescale ratio of Li-rich giants to normal giants was estimated using the population synthesis method. Then we obtained the theoretical value of A(Li) and made a comparison with observations. Results. Considering the influence of element diffusion in the model results in the increase of the lithium abundance up to about 1.8 dex, which can reveal Li-rich giants. Simultaneously, introducing high constant diffusive mixing coefficients (Dmix) with the values from 1011 to 1015 cm2 s−1 in the model allows the A(Li) to increase from 2.4 to 4.5 dex, which can explain most of the Li-rich and super Li-rich giant stars. The population synthesis method reveals that the amount of Li-rich giants is about 0.2−2% of all giants, which is consistent with observation estimated levels. Conclusions. In our models the element diffusion, mainly triggered by the gravity field, changes the mean molecular weight at the junction zone between the stellar envelope and the He core, which makes the thermohaline mixing region expand to the inner convection region of stars. A transport channel, efficiently transporting 7Be in the hydrogen-burning region of the star to the convective envelope where 7Be decays into 7Li, is formed. Combining high constant diffusive mixing coefficients, the transport channel can explain the origin of Li-rich and super Li-rich giants, even the most super Li-rich giants.