Li Enrichment in Red Clump Giants: Clues for Past Binary Interaction or Merger Events

Abstract To understand the underlying mechanisms of high Li abundances among core He-burning or red clump (RC) giants, we analyzed a sample of 5227 RC giants of mass M ≤ 2 M ⊙ using spectra and asteroseismic data. We found 120 RC giants (∼2%) with a lower limit of A (Li) = 0.7 dex, a factor of 40 more than their predecessors close to the red giant branch 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 the question of whether the binary interaction/merger is a prerequisite, along with the He flash, for Li enhancement among RC giants.