The 7Li isotope is one of the few species created during big bang nucleosynthesis (BBN), and it burns in stellar interiors via proton capture at relatively low temperatures. Other known astrophysical sources of Li include asymptotic giant branch (AGB) stars, novae, and cosmic ray spallation. During the evolutionary phases other than the AGB, single stars are not expected to add to the Li budget of the Galaxy, but to largely deplete it.
The pre-main sequence
Stars are born from an interstellar medium that has a Li content that: has increased from the BBN value over time, has remained roughly constant since the birth of the Sun, and which probably varies with galactocentric radius. Low-mass pre-MS stars are initially fully convective; as they contract their cores reach the Li ignition temperature and they rapidly destroy their Li content until a radiative zone forms in the core, pushes outwards and stops the mixing of Li-depleted material to the surface.
In standard theory this leads to a set of well-defined Li vs mass (or Teff) isochrones that should be composition-dependent1 and could in principle be used to estimate precise young stellar ages. The reality is different:
- There are spreads of Li at a given age and Teff.
- The dependence on composition appears to be weak or absent.
- Most (but not all) models predict too much pre-MS Li depletion.
- Rapid rotators are less depleted, also against theoretical expectation.
- Depletion continues from the ZAMS onwards throughout the MS, even though the convection zone base is too cool to burn Li.
These are fundamental problems that hamper the use of Li as a precise age indicator and probe of Galactic chemical evolution, pointing to a poor understanding of the interior physics of low-mass stars.
In recent years, a new family of solutions has been proposed, where Li depletion in the pre-MS is suppressed by the structural effects of rotation, magnetic fields, starspots, accretion, and even the presence of exoplanets. Thus, Li depletion in young stars connects to many facets of the Cool Stars community, and some of these may point the way to resolving the current challenges. As new generations of models incorporating these effects are being developed, new, homogeneous spectroscopic surveys are being combined with precise photometry and rotation periods to provide more precise and reliable benchmarks for testing the models.
The post-main sequence
The realization that most Li-rich giants are in the red clump phase has brought attention to the possible role of the core He flash in Li production. The idea that the He flash is capable of producing Li enrichment is further supported by the fact that Li-rich clump giants seem to be confined to masses below 2 solar masses2. However, the detailed physics of Li synthesis by the flash and the transport of that Li to the photosphere are both missing.
An equally serious complication is that the He flash occurs in all stars below about 2 solar masses, but Li-rich giants comprise only a few percent of red clump stars, and the large majority of clump giants have Li abundances that are consistent with standard stellar evolution. The He flash as the enrichment source for Li-rich giants would need an accompanying mechanism to explain the observed range in Li abundance and the small fraction of enriched stars at any given time.
Another possible mechanism for Li enrichment that has received attention is tidal interactions in a close binary system, where a companion would induce tides on the giant on the upper red giant branch, driving enough extra mixing to mimic the well-known Cameron-Fowler mechanism that occurs on the AGB. However, the few attempts to determine the binary fraction of Li-rich giants have found no differences with the larger Li-normal population. The promise of potentially new relevant physics for stellar interiors and nucleosynthesis makes now an opportune time to organize relevant discussions with the Cool Stars community.