CS 22964–161: A Double-Lined Carbon- and s-Process-Enhanced Metal-Poor Binary Star

A detailed high-resolution spectroscopic analysis is presented for the carbon-rich low-metallicity Galactic halo object CS 22964–161. We have discovered that CS 22964–161 is a double-lined spectroscopic binary and have derived accurate orbital components for the system. From a model atmosphere analysis we show that both components are near the metal-poor main-sequence turnoff. Both stars are very enriched in carbon and in neutron-capture elements that can be created in the s-process, including lead. The primary star also possesses an abundance of lithium close to the value of the “Spite plateau.” The simplest interpretation is that the binary members seen today were the recipients of these anomalous abundances from a third star that was losing mass as part of its AGB evolution. We compare the observed CS 22964–161 abundance set with nucleosynthesis predictions of AGB stars, discuss issues of envelope stability in the observed stars under mass transfer conditions, and consider the dynamical stability of the alleged original triple star. Finally, we consider the circumstances that permit survival of lithium, whatever its origin, in the spectrum of this extraordinary system.