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

TL;DR

This study analyzes lithium levels in low-mass red clump stars and finds no evidence supporting the need for a new, ubiquitous lithium production process during their evolution, aligning with standard stellar models.

Contribution

It demonstrates that observed lithium distributions in field red clump stars are consistent with existing stellar evolution models, negating the necessity for hypothesized new lithium production channels.

Findings

Observed Li abundances match standard model predictions.

Large numbers of low Li and upper limit measurements are confirmed.

Higher mass progenitors show higher Li abundances.

Abstract

Known sources of lithium (Li) in the universe include the big bang, novae, asymptotic giant branch stars, and cosmic ray spallation. During their longer-lived evolutionary phases, stars are not expected to add to the Li budget of the Galaxy, but to largely deplete it. In this context, recent analyses of Li data from GALAH and LAMOST for field red clump (RC) stars have concluded that there is the need for a new production channel of Li, ubiquitous among low-mass stars, and that would be triggered on the upper red giant branch (RGB) or at helium ignition. This is distinct from the "Li-rich giant" problem and reflects bulk RC star properties. We provide an analysis of the GALAH Li data that accounts for the distribution of progenitor masses of field RC stars observed today. Such progenitors are different than today's field RGB stars. Using standard post main-sequence stellar evolution, we show that the distribution of Li among field RC giants as observed by GALAH is consistent with standard model predictions, and does not require new Li production mechanisms. Our model predicts a large fraction of very low Li abundances from low mass progenitors, with higher abundances from higher mass ones. Moreover, there should be a large number of upper limits for RC giants, 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 RC stars, in concordance with our interpretation.