Lithium and rotation in F and G dwarfs and subgiants

Abstract

Lithium abundances have been determined in 127 F and G Pop I stars based on new measurements of the equivalent width of the 6707 Å Li I line from their high resolution CCD spectra. Distances and absolute magnitudes of these stars have been obtained from the Hipparcos Catalogue and their masses and ages derived, enabling us to investigate the behaviour of lithium as a function of these parameters. Based on their location on the HR diagram superposed on theoretical evolutionary tracks, the sample of the stars has been chosen to ensure that they have more or less completed their Li depletion on the main sequence. A large spread in the Li abundances is found at any given effective temperature especially in the already spun down late F and early G stars. This spread persists even if the "Li-dip" stars that have evolved from the main sequence temperature interval 6500-6800 K are excluded. Stars in the mass range up to 2 when divided into three metallicity groups show a linear correlation between Li abundance and mass, albeit with a large dispersion around it which is not fully accounted for by age either. The large depletions and the observed spread in Li are in contrast to the predictions of the standard stellar model calculations and suggest that they are aided by non-standard processes depending upon variables besides mass, age and metallicity. The present study was undertaken to examine, in particular, the effects of rotation on the depletion of Li. No one-to-one correlation is found between the Li abundance and the present projected rotational velocity. Instead the observed abundances seem to be dictated by the rotational history of the star. However, it is noted that even this interpretation is subject to the inherent limitation in the measurement of the observed Li EQW for large rotational velocities.