Lithium abundance and mass

Abstract

Observations of cool giants have shown that there exists a large range in their lithium abundances even for apparently similar stars. The depletions are large in a majority of them, far in excess of the predictions of the standard stellar evolution models. In order to explore whether the large spread in Li abundances observed in giants can be interpreted in terms of mass, moderately high resolution CCD spectra of the Li I line at 6707.8 Ä have been obtained in 65 subgiants, giants and supergiants and the lithium abundances derived. Their absolute magnitudes have been estimated from the Hipparcos data. Absolute magnitudes have also been determined for another 802 stars whose lithium abundances are already known from the available literature. All these stars have been plotted on the HR diagram and compared with the theoretical evolutionary tracks of Bressan et al. (1993) with initial masses ranging from 1 M_sun to 9 M_sun for a chemical composition typical of the solar neighbourhood: X=0.70, Y=0.28, Z=0.02. The stars of low mass of this sample, (<2M_sun), span a wide range in evolution (unmixed warm subgiants and mixed giants) and therefore, show a correspondingly wide range of Li abundances, perhaps reminiscent of the large range in abundances observed on the main sequence. The spread is further augmented by the effects of increasing dilution and mixing as the stars evolve to the right and up the red giant branch. Higher mass stars show a different behaviour. Many of the giants of masses between 2.5 and 4.0 M_sun observed in the present study have Li abundances close to what is predicted by the standard stellar models. On the other hand, there are several high mass giants (>2.5 M_sun) cooler than Teff = 5000 K with Li abundances as low as those of low mass stars of similar effective temperature. There must be parameters other than mass and evolutionary status, as implied by the standard evolution model of a star, that control its Li abundance.