Lithium in stellar atmospheres: observations and theory

Abstract

Of all the light elements, lithium is the most sensitive indicator of stellar evolution. This review discusses current data on the abundance of lithium in the atmospheres of A-, F-, G-, and K-stars of different types, as well as the consistency of these data with theoretical predictions. The variety of observed Li abundances is illustrated by the following objects in different stages of evolution: 1) Old stars in the galactic halo, which have a lithium abundance log e(Li)=2.2 (the "lithium plateau") that appears to be 0.5 dex lower than the primordial abundance predicted by cosmological models. 2) Young stars in the galactic disk, which have been used to estimate the contemporary initial lithium abundance log e(Li)=3.2 +/- 0.1 for stars in the Main sequence. Possible sources of lithium enrichment in the interstellar medium during evolution of the galaxy are discussed. 3) Evolving FGK dwarfs in the galactic disk, which have lower log e(Li) for lower effective temperature Teff and mass M. The "lithium dip" near Teff~6600 K in the distribution of log e(Li) with respect to Teff in old clusters is discussed. 4) FGK giants and supergiants, of which most have no lithium at all. This phenomenon is consistent with rotating star model calculations. 5) Lithium rich cold giants with log e(Li) > 2.0 , which form a small, enigmatic group. Theoretical models with rotation can explain the existence of these stars only in the case of low initial rotation velocities V0 < 50 km/s. In all other cases it is necessary to assume recent synthesis of lithium (capture of a giant planet is an alternative). 6) Magnetic Ap-stars, where lithium is concentrated in spots located at the magnetic poles. There the lithium abundance reaches log e(Li)=6. Discrepancies between observations and theory are noted for almost all the stars discussed in this review.