Isotopic Lithium Abundances in Nine Halo Stars

Open Access
Publisher Website
Google Scholar
Abstract
High-resolution echelle spectra of the 6707.8 Å Li I absorption line in spectra of metal-poor halo dwarfs and subgiants are analyzed for the isotopic fractions of 6Li and 7Li. Positive detections of 6Li are reported for two stars, HD 84937 and BD +26° 3578, and tight upper limits to the 6Li fraction are obtained for an additional seven stars. Two stars, HD 116064 and BD +26° 2606, were found to be double-lined spectroscopic binaries and were rejected from the analysis. Elemental Li abundances were also derived for the nine program stars: the Li abundance is identical across the sample (excluding the spectroscopic binaries) to within ±0.04 dex in log (Li). Two processes involving high-energy particles (cosmic rays) are considered as origins of the 6Li: spallation reactions (e.g., O + p →6Li) and α-α fusion reactions. The contribution of spallation reactions may be assessed empirically using the published Be and B abundances. The observed 6Li/Be ratio is significantly in excess of that expected from spallation. In view of the fact that the surface 6Li abundance has almost certainly been somewhat depleted by exposure to warm protons, the conclusion is that α-α reactions produced most of the observed 6Li. Consequences of this conclusion are sketched.