Lithium Abundance of Halo Stars With New T_eff

Extract

The lithium observed in the atmospheres of unevolved halo stars is believed to be an essentially unproccessed element which reflects the primordial yields. The primordial nature of Li is inferred from the presence of a constant Li abundance for all the halo dwarfs where convection is not effective (T_eff ≥ 5600 K). Such an uniformity is taken as evidence for the absence of any stellar depletion during the formation and the long life of the halo stars and also as evidence for the absence of any production mechanism acting either before or at the same time of the formation of the halo population. The existence of a real plateau has been recently questioned by Thorburn (1994), Norris et al (1994) and Deliyannis et al (1993). These results open the possibility of substantial depletion by rotational mixing where a certain degree of dispersion is foreseen for different initial angular momenta of the stars and/or to a significant Galactic Li enrichment within the first few Gyrs.

Lithium abundances in a sample (39) of halo dwarfs have been redetermined by using the new T_eff derived by Fuhrmann et al (1994) from modelling of the Balmer lines. These T_eff are reddening independent, homogeneous and of higher quality than those based on broad band photometry. The sample represent a significant fraction of the presently available Li determinations, and form an unique sample with a good and homogeneous T_eff Fuhrmann et al are also able to provide errors in the T_eff for individual stars and in most cases they are as good as ± 50 K, which is a factor 2 smaller than the grossly estimated errors for photometric-based T_eff. The Li equivalent widths have been taken from the literature and the theoretically derived random errors from Deliyannis et al (1993) or computed following their prescriptions. Abundances have been derived by generating new atmospheric models by using the ATLAS-9 code by Kurucz (1993) with enhanced α-elements and without the overshooting option. If we use the Kurucz 1993 grid which includes overshooting the Li abundance is increased by ≈ 0.08 dex.