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

Abstract

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) having interesting implications for Big Bang nucleosynthesis, or the extraordinary action of Galactic Li production mechanisms. It is also possible that both mechanisms have acted. We use our high resolution, high S N Keck HIRES spectrum of BD+23 3912 to determine the n-capture abundances and 6Li 7Li ratio in this star. These values serve as signatures for two possible Li production scenarios: the 7Be transport mechanism in AGB stars and cosmic ray interactions with the ISM. The unremarkable abundances of Y, Zr, Ba, La, Nd, and Sm that we derive agrue against a significant contribution to this star's excess Li from AGB production mechanisms carrying an s-process signature. Our conservative upper limit of 6Li 7Li ≤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 6Li 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. While the high Li abundance in BD+23 3912 is consistent with that expected from Yale rotational models having a lower-than-average initial angular momentum, future observations of ν-process elements (particularly 11B) produced in supernovae should provide additional constraints on any enrichment scenarios seeking to explain the large Li abundance of this interesting star.