Lithium Abundances in Giants as a Function of Stellar Mass: Evidence for He Flash as the Source of Li Enhancement in Low-mass Giants

Abstract

In this work, we studied the distribution of lithium abundances in giants as a function of stellar mass. We used a sample of 1240 giants common among Kepler photometric and LAMOST medium-resolution (R ≈ 7500) spectroscopic survey fields. The asteroseismic ΔP–Δν diagram is used to define core He-burning red clump giants and red giant branch stars with an inert He core. Li abundances have been derived using spectral synthesis for all sample stars. Directly measured values of asteroseismic parameters ΔP (or ΔΠ1) and Δν are either taken from the literature or measured in this study. Of the 777 identified red clump giants, we found 668 low-mass (≤2 M ⊙) primary red clump giants and 109 high-mass (>2 M ⊙) secondary red clump giants. Observed Li abundances in secondary red clump giants agree with the theoretical model predictions. The lack of Li-rich giants among secondary red clump giants and the presence of Li-rich, including super Li-rich, giants among primary red clump stars reinforces the idea that helium flash holds the key for Li enrichment among low-mass giants. The results will further constrain theoretical models searching for a physical mechanism for Li enhancement among low-mass red clump giants. Results also serve as observational evidence that only giants with mass less than ≈2 M ⊙ develop a degenerate He core and undergo He flash.