The physical mechanism for the formation of lithium-rich red clump stars: Rotation, thermohaline mixing, and internal gravity waves

¹ College of Physics, Guizhou University, Guiyang, Guizhou Province 550025, PR China
² Geneva Astronomical Observatory, Geneva University, Sauverny CH-1290, Switzerland
³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
⁴ College of Physics, Henan Normal University, Xinxiang, Henan Province 453007, PR China
⁵ Department of Physics, Anhui Normal University, Wuhu 241000, PR China

^⋆ Corresponding author; hfsong@gzu.edu.cn, georges.meynet@unige.ch

Received: 22 September 2024
Accepted: 30 November 2024

Abstract

Context. About 0.2–2% of red clump stars are revealed as Lithium-rich stars and thus the surface abundance of lithium clearly increases in some red clump stars. The physical mechanism of the enrichment of lithium on the surface of these stars has not yet been explained satisfactorily by the evolutionary models of single stars.

Aims. Our aim is to investigate how rotation, thermohaline mixing, and internal gravity waves have an important impact on the surface chemical abundance of lithium-rich red giants.

Methods. The equations for angular momentum transport and the chemical element diffusion for rotating stars have been implemented in this paper. The diffusion coefficients of rotationally induced instabilities, thermohaline mixing, and internal gravitational waves have been included in the diffusion equation of chemical elements.

Results. Rotational mixing, thermohaline mixing, and internal gravity waves have been invoked to explain this feature. Rotation impacts the evolution of the surface abundance of Lithium, but it seems an unlikely explanation for a ubiquitous mixing event occurring between the tip of the red giant star and the red clump star. Thermohaline mixing can explain the observed behaviour of ¹²C/¹³C and ¹⁴N/¹²C and lithium in low-mass stars that are more luminous than the red-giant branch bump, and its efficiency is decreasing with the increasing initial stellar mass.

Conclusions. The internal gravity wave- (IGW-) induced mixing is located between the hydrogen-burning shell, and the outer convective envelope, and it is mainly triggered by turbulent convective motion. This physical process is beneficial to transfer the large amount of ⁷Be to the cool envelope where it is converted to ⁷Li. Therefore, IGW-induced mixing could play a main role in explaining the red clump star with lithium enrichment. Rotation can indirectly increase the above effect by making the core-helium-burning lifetime longer. Thermohaline mixing is much smaller than the one of IGWs during the evolution of red clump stars.