A low upper-limit on the lithium isotope ratio in HD140283

We have obtained a high-S/N (900-1100), high-resolving-power (R= 95 000) spectrum of the metal-poor subgiant HD 140283 in an effort to measure its ⁶Li/⁷Li isotope ratio. From a 1-D atmospheric analysis, we find a value consistent with zero, ⁶Li/⁷Li = 0.001, with an upper limit of ⁶Li/⁷Li < 0.018. This measurement supersedes an earlier detection (0.040 ± 0.015(1σ)) by one of the authors. HD 140283 provides no support for the suggestion that Population II stars may preserve their ⁶Li on the portion of the subgiant branch where ⁷Li is preserved. However, this star does not defeat the suggestion either; being at the cool end of the subgiant branch of the Spite plateau, it may be sufficiently cool that ⁶Li depletion has already set in, or the star may be sufficiently metal poor that little Galactic production of ⁶Li had occurred. Continued investigation of other subgiants is necessary to test the idea. We also consider the implications of the HD 140283 upper limit in conjunction with other measurements for models of ⁶Li production by cosmic rays from supernovae and structure formation shocks.

Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.