Lithium as a probe of stellar and galactic physics

Lithium plays a unique role in astrophysics, as it is a powerful diagnostic for the physics and evolution of low-mass stars, Galactic archaeology, and cosmology. We review the Li observations in stars at different phases of their evolution, the strengths and the limitations of the current theoretical stellar models to explain the Li abundance data, our understanding of the Li sources and of the evolution of Li through- out the Galactic history. Key takeaways from the current state of the research in the field are: 1) Stellar evolution models accounting for fundamental transport processes of chemical species and angular momentum hold the promise of providing a common stellar Li depletion explanation to the Li abundance patterns observed in all Galactic stellar populations, including the dip and the plateau(s). 2) Novae are most probably the main source of Li in the Galaxy, on observational (but not yet theoretically established) grounds. 3) Radial migration of stars in the Galactic disk holds the key to understand many aspects of the Li evolution in the Milky Way.

💡 Research Summary

Lithium occupies a singular position in astrophysics because it simultaneously probes stellar interiors, Galactic chemical evolution, and primordial nucleosynthesis. This review synthesises the wealth of observational data on lithium abundances across a broad range of stellar evolutionary phases and evaluates the capacity of current theoretical models to reproduce these data.

The authors begin by recalling the historic role of lithium as a diagnostic of stellar structure. Early work showed that the Sun’s photospheric lithium is depleted by about two orders of magnitude relative to meteoritic values, implying transport of material from the cool surface to hotter interior layers where lithium is destroyed. Subsequent observations of open‑cluster dwarfs, solar analogues, and halo stars revealed systematic trends: a temperature‑dependent “Li‑dip” around 6600 K, a near‑constant “Spite plateau” in metal‑poor halo dwarfs, and a large scatter among field stars of similar mass and age. Moreover, a small fraction of red‑giant stars exhibit anomalously high lithium (“Li‑rich giants”), a phenomenon that cannot be explained by standard convection alone.

To account for these patterns, the review discusses five families of non‑standard stellar physics. Atomic diffusion alone can produce modest surface lithium declines but fails to generate the sharp dip. Rotational mixing (type‑I models) introduces shear‑induced transport that deepens the lithium‑burning region, reproducing part of the dip and the age‑dependent depletion seen in solar‑type stars. Type‑II models add magnetic instabilities, further enhancing mixing efficiency and better matching the observed depth and metallicity dependence of the dip. Magneto‑inertial gravity waves provide an additional angular‑momentum transport channel that can explain the existence of Li‑rich giants by dredging up freshly synthesized lithium during the red‑giant branch. The authors stress that no single mechanism reproduces all observations; a combination of diffusion, rotation, magnetic fields, and wave‑driven transport appears necessary.

Lithium production is examined in five astrophysical sites. Primordial nucleosynthesis creates most of the Universe’s 7Li, but the predicted abundance exceeds the Spite plateau, leading to the “lithium problem”. Cosmic‑ray spallation and α‑α fusion generate both 6Li and 7Li, contributing modestly to the Galactic budget. Asymptotic Giant Branch (AGB) stars can produce lithium via hot‑bottom burning and the Cameron‑Fowler mechanism, yet their overall contribution is limited by short lifetimes and uncertain mass‑loss rates. Classical novae, through explosive hydrogen burning, have emerged as the most promising stellar source; recent spectroscopic detections of 7Be (which decays to 7Li) in nova ejecta suggest that novae may supply 30–50 % of the present‑day Galactic lithium. Finally, the ν‑process in core‑collapse supernovae can synthesize a small amount of lithium, but its impact on the Galactic inventory is minor.

The review then integrates lithium evolution into Galactic chemical evolution (GCE) models. In the solar neighbourhood, the observed rise of lithium with metallicity can be reproduced only when both novae and low‑mass stellar sources are included, together with a realistic delay‑time distribution for nova eruptions. The thin‑disk lithium trend is strongly shaped by radial migration: stars born in the inner, metal‑rich regions migrate outward, carrying higher lithium abundances to the solar circle, while outward‑born stars bring lower lithium inward, smoothing the overall trend. In the thick disk and bulge, older stellar populations exhibit lower lithium at a given metallicity, consistent with a reduced contribution from novae and a shorter timescale for chemical enrichment. The evolution of the 6Li/7Li isotopic ratio further constrains the relative importance of cosmic‑ray spallation versus stellar sources.

Key take‑aways are: (1) modern stellar evolution models that incorporate angular‑momentum transport (rotation, magnetic instabilities, gravity waves) and atomic diffusion are on the verge of providing a unified explanation for the lithium dip, plateau, and red‑giant depletion patterns. (2) Observational evidence points to novae as the dominant contemporary lithium producer, but theoretical yields and frequency remain uncertain. (3) Radial migration of stars within the Galactic disk is essential to reconcile local lithium observations with global GCE predictions. The authors conclude that forthcoming high‑precision asteroseismic ages, large‑scale spectroscopic surveys (e.g., Gaia‑ESO, GALAH, 4MOST), and improved nova nucleosynthesis models will be decisive in resolving the remaining discrepancies and fully exploiting lithium as a probe of stellar and Galactic physics.