Strong lithium lines in red supergiants at different metallicities

TL;DR

This study finds that many red supergiants, especially in lower metallicity environments like the Magellanic Clouds, retain or produce lithium in their atmospheres, challenging existing stellar evolution models.

Contribution

It provides the first large-scale spectroscopic survey showing significant lithium presence in red supergiants across different metallicities, suggesting a need to revise stellar evolution theories.

Findings

Approximately one third of Perseus Arm RSGs show lithium lines.

High lithium detection rate (~40%) in LMC clusters.

Lower detection rate in SMC clusters, indicating metallicity dependence.

Abstract

Current models of stellar evolution predict that stars more massive than $\sim6\:$M$_{\odot}$ should have completely depleted all lithium (Li) in their atmospheres by the time when they reach the He core burning phase. Against this, a non-negligible number of red giants with masses $>6\:$M$_{\odot}$ presenting strong Li lines have recently been reported. Motivated by this finding, we have carried out a spectroscopic survey of red supergiants (RSGs) in the Perseus Arm and a selection of young open clusters in the Magellanic Clouds to assess the presence of the Li I $\lambda$6708 doublet line. Based on a sample of >70 objects, close to one third of RSGs in the Perseus Arm display noticeable Li lines, with perhaps a trend towards a lower fraction among more luminous stars. The samples in the Magellanic Clouds are not as large, but hint at a metallicity dependence. Twenty one RSGs in 5 LMC clusters show a very high fraction of Li detection, around 40%. Conversely, 17 RSGs in 5 SMC clusters give only one secure detection. The interpretation of these observational results is not straightforward, but a mechanism for Li production seems most likely. Further characterisation work is ongoing, while theoretical studies into this matter are urgently needed.