Lithium abundance and stellar rotation in the Galactic halo and thick disc: Contribution from low-mass giant field stars

Abstract

The stellar evolution of lithium-rich (Li-rich) giant stars at very low metallicities remains largely unexplored to date. Using two recent large LAMOST catalogues of field, low-mass giant stars (both Li-rich and Li-poor) with metallicities from -4.0 to -1.0, we studied the conditions for Li enrichment and the 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 RGB stars are known in this regime. The observational appearance of giants across the horizontal branch (HB) and asymptotic giant branch (AGB) stages (with Li abundances up to 6.15 dex) has been detected for metallicities > -2.5. Among these stars, we detected IR excesses 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 with IR excesses are distributed across most metallicity values, we suggest this mechanism is at work throughout an important part of the Galaxy's evolutionary history. Based on these IR excesses, we identified three Li thresholds: about 1.5 dex for RGB stars, about 0.5 dex for HB stars, and about -0.5 dex for AGB stars, establishing a new criterion to characterise Li-rich giants. We carried out a study of stellar rotations in metal-poor giant stars, revealing that a plateau appears for velocities greater than 40 km/s up to near 90 km/s, with Li abundances from 1.02 to 1.82 dex. Among Li-rich giants with v sin i > 40 km/s, increasing rotation is observed as metallicity decreases from -1.0 to -2.5. The presence of RGB and HB Li-rich giants with rotations up to 90 km/s suggests that stellar models must account for extended 3He reservoir lifetimes as a source of 7Li. The velocity around 40 km/s appears to be a new critical value.