Mass Matters: No Evidence for Ubiquitous Lithium Production in Low-Mass Clump Giants

Spectroscopic surveys are providing large samples of lithium (Li) measurements in evolved stars. A seemingly unexpected result from this work has been the apparent detection of Li at a higher rate in core helium-burning stars than in luminous shell hydrogen-burning stars, which has been interpreted as evidence for ubiquitous Li production on the upper red giant branch or at helium ignition. This is distinct from the "Li-rich giant" problem and reflects bulk red clump star properties. We provide an analysis of the GALAH Li data that accounts for the distribution of progenitor masses of field red clump stars observed today. Using standard models of the post-main sequence evolution of low-mass stars, we show that the observed distribution of Li among the bulk of field clump giants is natural, and that observations of clump and red giants in the field should not be compared without correcting for population effects. For typical stellar population distributions, moderate Li abundances among the bulk of field clump giants are expected without the need for a new Li production mechanism. Our model predicts a large fraction of very low Li abundances from low mass progenitors, with higher Li abundances from higher mass ones. Moreover, there should be a large number of upper limits for red clump stars, and higher abundances should correspond to higher masses. The most recent GALAH data indeed confirm the presence of large numbers of upper limits, and a much lower mean Li abundance in clump stars, which is concordant with our interpretation.