Extremely Metal-poor Stars. VII. The Most Metal-poor Dwarf, CS 22876–032

We report high-resolution, high signal-to-noise observations of the extremely metal-poor double-lined spectroscopic binary CS 22876-032. The system has a long period: P = 424.7 ± 0.6 days. It comprises two main-sequence stars having effective temperatures 6300 and 5600 K, with a ratio of secondary to primary mass of 0.89 ± 0.04. The metallicity of the system is [Fe/H] = -3.71 ± 0.11 ± 0.12 (random and systematic errors)—somewhat higher than previous estimates. We find [Mg/Fe] = 0.50, typical of values of less extreme halo material. [Si/Fe], [Ca/Fe], and [Ti/Fe], however, all have significantly lower values, ~0.0-0.1, suggesting that the heavier elements might have been underproduced relative to Mg in the material from which this object formed. In the context of both the hypothesis that the abundance patterns of extremely metal-poor stars are driven by individual enrichment events and the models of Woosley & Weaver, the data for CS 22876-032 are consistent with its having been enriched by a zero-metallicity supernova of mass 30 M☉. As the most metal-poor near-main-sequence turnoff star currently known, the primary of the system has the potential to strongly constrain the primordial lithium abundance. We find A(Li) {= log[N(Li)/N(H)] + 12.00} = 2.03 ± 0.07, which is consistent with the finding of Ryan et al. that, for stars of extremely low metallicity, A(Li) is a function of [Fe/H].