On the observational characteristics of lithium-enhanced giant stars in comparison with normal red giants

TL;DR

This study investigates the observational features of lithium-rich giant stars compared to normal giants, revealing differences in rotation and elemental abundances, and suggesting that Li enhancement is likely a transient evolutionary phase rather than planet engulfment.

Contribution

It provides a detailed spectroscopic comparison of Li-rich and normal giants, highlighting their similarities and differences, and discusses potential origins of Li enrichment.

Findings

Li-rich giants show larger rotational velocities.

Li enrichment may be a transient evolutionary episode.

Li-rich giants exhibit Be deficiency and lack of 6Li.

Abstract

With an aim to shed light on the origin of Li anomaly observed in a small fraction of red giants, we carried out a spectroscopic study on the observational characteristics of selected 20 bright Li-rich giants, in comparison with a large number of normal late G-early K giants, where special attention was paid to clarifying any difference between two samples with respect to stellar parameters, rotation, activity, kinematic properties, and chemical abundances of Li, Be, C, O, Na, S, and Zn). Our targets are roughly divided into 'bump/clump group' and 'luminous group' according to the position on the HR diagram. Regarding the former group, Li-enriched giants and the corresponding normal giants appear practically similar except for Li, suggesting that surface Li enhancement in this group may be a transient episode which normal giants undergo at some certain evolutionary stage. Meanwhile, those Li-rich giants belonging to the latter group appear more anomalous in the sense that they tend to show larger rotational velocities, and that their elemental abundances (e.g., those of C and O derived from high-excitation lines) are apt to show apparent overabundances, though this might be due to some spurious effect reflecting the difficulty of abundance derivation in stars of higher rotation and activity. Our analysis confirmed considerable Be deficiency as well as absence of 6Li as the general characteristics of Li-rich giants under study, which implies that engulfment of planets is rather unlikely for the cause of Li-enrichment.