Lithium abundance and stellar rotation in the Galactic halo and thick disc

Abstract

The stellar evolution of lithium-rich (Li-rich) giant stars at very low metallicities remains largely unexplored to date. Using mainly two recent large LAMOST catalogues of field, low-mass giant stars (both Li-rich and Li-poor) with metallicities ranging from −4.0 up to −1.0, we studied some of the conditions for Li enrichment and the general distribution of stellar rotations in the Galactic halo and thick disc. Due to the scarcity of stars with [Fe/H] < −3.0, only three Li-rich red giant branch (RGB) stars are known in this regime. The full observational appearance of all giants, across the horizontal branch (HB) and asymptotic giant branch (AGB) advanced stages of evolution (with Li abundances up to 6.15 dex) have been detected for metallicities > −2.5. Among these stars, we detected the presence of IR excesses that are considered to be indicative of giant stars losing mass, showing a recent episodic Li-enrichment process related to the Cameron-Fowler mechanism for the formation of new 7Li. Because stars presenting IR excesses are distributed across the majority of metallicity values, we suggest this mechanism is at work throughout an important part of the evolutionary history of the Galaxy. Based on these IR excesses, we identified three Li thresholds: ∼1.5 dex for RGB stars, ∼0.5 dex for HB stars, and approximately −0.5 dex for AGB stars, thereby establishing a new criterion to characterise Li-rich giants in the halo and thick disc. We carried out a first extensive study of stellar rotations in metal-poor giant stars, revealing the following results: (a) a plateau appears for velocities greater than 40kms−1 extending up to near 90kms−1, with Li abundances ranging from 1.02 to 1.82 dex; (b) among Li-rich giant stars with vsini > 40kms−1, a clear trend toward increasing rotation is observed up to near 90kms−1, as metallicities decrease from −1.0 to −2.5, it is observed; (c) the presence of RGB and HB Li-rich giant stars with rotations up to 90kms−1 suggests that internal stellar models must account for extended 3He reservoir lifetimes as a source of 7Li considering these velocities. The velocity around 40kms−1 appears to be a new critical value that merits further investigation.