Constraints on the origin of the remarkable lithium abundance in the halo star BD+23 3912

The Li abundance of the halo star BD+23 3912 ([Fe/H]= -1.5) lies a factor of 2-3 above the Spite plateau. This remarkable difference could reflect either less-than-average stellar Li depletion from a higher primordial Li abundance (as predicted by the Yale rotational stellar evolutionary models), which may have interesting implications for Big Bang nucleosynthesis, or the extraordinary action of Galactic Li production mechanisms. It is also possible that both processes have acted. We use our high resolution, high S/N Keck HIRES spectrum of BD+23 3912 to determine the s-process element abundances and Li-6/Li-7 ratio in this star. These values serve as signatures for two possible Li production scenarios: the Be-7 transport mechanism in AGE stars, and cosmic ray interactions with the ISM. The unremarkable abundances of Y, Zr, Ba, La, Nd, and Sm that we derive argue against a significant contribution to this star's excess Li from AGE production mechanisms carrying an s-process signature. Since halo subgiants like BD+23 3912 are expected to be particularly good Li-6 preservers, our conservative upper Limit of Li-6/Li-7 less than or equal to 0.15 (compared to 0.25-0.50 expected from cosmic ray production) argues against cosmic ray + ISM interactions as the source for the excess Li, unless Li depletion from an even higher abundance has occurred with preferential Li-6 depletion. Highly speculative RGB production scenarios also seem unlikely given the normal Na and Al abundances we find and the normal C and O abundances determined by others. The totality of Li data on halo subgiants argues against possible diffusion scenarios, in which all such stars dredge up Li that diffused during the main sequence. While the high Li abundance in BD+3912 is consistent with that expected from Yale rotational models having a lower-than-average initial angular momentum, future observations of nu-process elements (particularly B-11) produced in supernovae should provide additional constraints on any enrichment scenarios seeking to explain the large Li abundance of this interesting star. (C) 1996 American Astronomical Society.