Extremely metal poor stars. VI. The heterogeneous class of lithium-depleted main-sequence turnoff dwarfs

We present abundances of 14 elements in the metal-poor main-sequence turnof f star G122-69, which has at most one-tenth the Li abundance observed in mo st other stars of similar temperature and metallicity. The deficiency of Li is significant because of this element's role in constraining primordial f nucleosynthesis and the baryon density of the universe. Although we have ex amined elements of intermediate atomic mass, in the iron peak, and heavy ne utron capture species, we find no other abundance anomalies in G122-69. Nor do we find any evidence of radial velocity variation at the 1 km s(-1) lev el. These data are combined with our previous study of the three other known Li -deficient main-sequence turnoff stars, thus permitting an analysis of the full sample. The new data reinforce our earlier finding that there is no ob vious common abundance abnormality that one might associate with the Li def iciency. Indeed, the four stars exhibit diverse abundance patterns and form a heterogeneous group. That said, the other three members of the group appear to have higher value s of [Ba/Sr], but not necessarily of [Ba/Fe] or [Sr/Fe], than most "normal" halo or even metal-deficient barium stars. The higher than average [Ba/Sr] ratios may indicate that their envelope material underwent s-processing wi th a high neutron exposure near the limit of that identified in the metal-d eficient barium stars and Population I analogs, which are thought to origin ate through contamination by asymptotic giant branch star ejecta. However, we cannot favor such an explanation any more than a normal r-process origin , especially in view of the unremarkable [Sr/Fe] and [Ba/Fe] values (except perhaps in G186-26). The most we can say is that the [Ba/Sr] ratios in thr ee of these stars are at the high end of the range encountered for "normal" halo stars, but that no mechanism has been unambiguously identified as res ponsible for their Li-depletion.