Meridional Circulation. I. A Formation Channel for Lithium-rich and Super Lithium-rich Red Clump Stars

TL;DR

This paper models how meridional circulation in rotating red clump stars can transport beryllium into the envelope, leading to lithium enrichment and explaining the formation of lithium-rich and super lithium-rich red clump stars.

Contribution

It introduces a diffusion-based model of meridional circulation to explain lithium enrichment in rotating red clump stars, linking rotation velocity to surface lithium levels.

Findings

Meridional circulation can increase lithium abundance by up to 3.0 dex.

Faster rotation correlates with higher surface lithium enrichment.

Lithium-rich red clump stars are predicted to be relatively young (~10^6 years).

Abstract

Current observations indicate that stars with higher rotation rates appear to maintain more surface lithium, and the majority of lithium-rich giants are indeed red clump stars. Hence, we investigate the mechanisms behind lithium enrichment in rotating red clump stars and the pathways to forming lithium-rich red clump stars. Meridional circulation is prevalent in the radiative zone of rotating giants. We model its radial mixing as a diffusion process and derive the corresponding diffusion coefficient based on its material transfer effect. Due to uncertainties in numerical calculations, we consider an average diffusion effect. Additionally, certain limiting conditions for the radial velocity of meridional circulation are incorporated. With varying input rotation velocities, we simulate the lithium evolution for red clump stars with this model. Our results indicate that the material transfer effect due to meridional circulation can efficiently transport beryllium, produced by H burning, into the convective envelope. This meridional circulation can lead to lithium enrichment, with a maximum lithium abundance increment approaching \(3.0\,\rm dex\). Consequently, it is capable of forming both lithium-rich and super lithium-rich red clump stars. The degree of lithium enrichment exhibits a strong positive correlation with the rotation velocity, i.e., faster red clump stars show more surface lithium. Furthermore, our models indicate that lithium-rich red clump stars are relatively young (\(\sim 10^6\,\rm yr\)), which aligns with observation evidences.