Lithium in Open Cluster Red Giants Hosting Substellar Companions

TL;DR

This study investigates whether substellar companions influence lithium content in red giants within open clusters, finding some cases of lithium enrichment but no definitive link to the presence of companions due to multiple influencing factors.

Contribution

It provides the first detailed comparison of lithium and carbon isotope ratios in red giants with substellar companions within open clusters, highlighting the complexity of factors affecting lithium abundance.

Findings

NGC2423 3 is Li-rich with A(Li)_NLTE=1.56 dex.

Substellar host stars generally have higher A(Li) and 12C/13C than control stars.

High lithium levels are not conclusively linked to substellar companions.

Abstract

We have measured stellar parameters, [Fe/H], lithium abundances, rotation, and 12C/13C in a small sample of red giants in three open clusters that are each home to a red giant star that hosts a substellar companion (NGC2423 3, NGC4349 127, and BD+12 1917 in M67). Our goal is to explore whether the presence of substellar companions influences the Li content. Both 12C/13C and stellar rotation are measured as additional tracers of stellar mixing. One of the companion hosts, NGC2423 3, is found to be Li-rich with A(Li)_NLTE=1.56 dex, and this abundance is significantly higher than the A(Li) of the two comparison stars in NGC2423. All three substellar companion hosts have the highest A(Li) and 12C/13C when compared to the control red giants in their respective clusters; however, except for NGC2423 3, at least one control star has similarly high abundances within the uncertainties. Higher A(Li) could suggest that the formation or presence of planets plays a role in the degree of internal mixing on or before the red giant branch. However, a multitude of factors affect A(Li) during the red giant phase, and when the abundances of our sample are compared to abundances of red giants in other open clusters available in the literature, we find that they all fall well within a much larger distribution of A(Li) and 12C/13C. Thus, even the high Li in NGC2423 3 cannot be concretely tied to the presence of the substellar companion.