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

TL;DR

This study analyzes Li-rich giant stars in the Galactic halo and thick disc across metallicities from -4.0 to -1.0 using two large LAMOST catalogs, focusing on Li enrichment mechanisms and stellar rotation. It provides evidence that the Cameron-Fowler mechanism operates across RGB, HB, and AGB phases, evidenced by IR-excess linked mass-loss episodes and three Li-abundance thresholds, and reveals a rotation- Li connection with a plateau for $v\sin i > 40$ km s^-1 and metallicity-dependent trends among Li-rich rotators. The findings imply universal Li production tied to angular-momentum evolution, require extended lifetimes for the $^3$He reservoir in models, and highlight the need for larger, more complete samples at [Fe/H] < -3 to quantify Li production contributions to Galactic chemical evolution. The work has significant implications for modelling Li enrichment history in the early Galaxy and for interpreting Li abundances in metal-poor giants.

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.

Figures (9)

• Figure 1: Diagram of Toomre for the stellar samples used in this work. Green indicates AGB stars, red represents HB stars, and blue corresponds to RGB stars. The two semicircles indicate, as a first approximation, the boundaries between the thin and thick disc (70 km s$^{-1}$), and between the thick disc and the halo (180 km s$^{-1}$).
• Figure 2: [$\alpha$/Fe] versus [Fe/H]. Most of the stars are concentrated around [$\alpha$/Fe] $=$ 0.23 (median) with a $\sigma =$ 0.07. Green indicates AGB stars, red represents HB stars, and blue corresponds to RGB stars.
• Figure 3: Spatial distribution of the giant stars used in this work. Here, $\omega_{\mathrm{GC}}$ is the Galactocentric radius projected onto the Galactic plane and $|Z|$ is the absolute distance from the mid-plane. The radial extent of each stellar class increases as $\log g$ decreases. Colours and symbols are the same as in Fig. \ref{['fig:Toomre']}.
• Figure 4: Li abundance as a function of $T_{\text{eff}}$. See the main text for details.
• Figure 5: Li abundance trends in evolved stars. Panel a (top): General stellar distributions of the catalogues of Ding24 and Gao21, showing Li abundances as a function of $\log g$. The horizontal line at A(Li)=1.5 serves only as a reference, while the vertical line marks the end of the stellar giant branch at $\log g = 3.5$. Panel b (bottom): Li abundance of Li-rich giant stars, RGB (blue), HB (red), and AGB (green), as a function of metallicity ([Fe/H]) in the range $-4.0$ to $-1.0$. While RGB stars are present throughout the metallicity range, HB and AGB stars appear only for [Fe/H] $> -2.5$.