Li-enrichment in red clump giants: Clues for past binary interaction or merger events

TL;DR

This study investigates lithium enrichment in red clump giants, linking high lithium levels and rotation to past binary interactions or mergers, suggesting a possible common origin for these phenomena.

Contribution

It provides evidence that binary interactions or mergers may be essential for lithium enhancement in red clump giants, a novel insight into stellar evolution processes.

Findings

High lithium abundance found in ~2% of RC giants.

Most high rotation RC giants are also Li-rich, with rotation declining as Li decreases.

Significant occurrence of Li-rich giants among low-mass and anomalous [C/N] ratio stars.

Abstract

To understand the underlying mechanisms for high lithium abundances among core He-burning or red clump (RC) giants, we analyzed a sample of 5227 RC giants of mass M $\leq$ 2~M$_{\odot}$ using spectra and asteroseismic data. We found 120 RC giants ($\sim$2~$\%$) with a lower limit of A(Li) = 0.7~dex, a factor of 40 more than their predecessors close to the RGB tip. Of the 120 RC giants, we could measure actual rotations for 16 RC giants using stellar spots from the Kepler light curve analysis. We found that most of the high rotation RC giants are also very high Li-rich RC giants, and the rotation seems to decline rapidly with Li abundance depletion, suggesting that both the high rotation and high Li abundance are transient phenomena and associated with a single source. Further, we found a significantly high occurrence of 15~$\%$ and 12~$\%$ of Li-rich RC giants among extremely low-mass RC giants and RC giants with anomalous [C/N] ratios, respectively. The extremely low mass, fast rotation and anomalous [C/N] values of RC giants are attributed to their past binary interaction/merger history. The results pose a question of whether the binary interaction/merger is a prerequisite along with the He-flash for Li-enhancement among RC giants.