Lithium in Kepler Red Giants: Defining Normal and Anomalous

Abstract

The orders-of-magnitude variations in the lithium abundances of evolved stars have long been a puzzle. Diluted signals, ambiguous evolutionary states, and unknown masses have made it challenging to both map the expected lithium signals and explain the anomalously lithium-rich stars. Using a set of asteroseismically characterized evolved stars, we show here that the base lithium abundance in red giant stars is mass-dependent, with higher-mass stars having higher “normal” lithium abundances, while highly lithium-enhanced stars may cluster around 0.8 or 1.8 M ☉. We confirm previous studies that have shown that lithium enhancement and rapid rotation are often coincident but find that the actual correlation between lithium abundance and rotation rate, whether surface, internal, or radial differential rotation, is weak. Our data support previous assertions that most lithium-rich giants are in the core-helium-burning phase. We also note a tentative correlation between the highest lithium abundances and unusual carbon-to-nitrogen ratios, which is suggestive of binary interactions, though we find no simple correlation between lithium richness and indicators of binarity.