Extremely metal-poor stars. VII. The most metal-poor dwarf, CS 22876-032

We report high-resolution, high signal-to-noise observations of the extreme ly metal-poor double-lined spectroscopic binary CS 22876-032. The system ha s a long period: P = 424.7 +/- 0.6 days. It comprises two main-sequence sta rs 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 th an previous estimates. We find [Mg/Fe] = 0.50, typical of values of less extreme halo material. [S i/Fe], [Ca/Fe], and [Ti/Fe], however, all have significantly lower values, similar to 0.0-0.1, suggesting that the heavier elements might have been un derproduced relative to Mg in the material from which this object formed. I n the context of both the hypothesis that the abundance patterns of extreme ly metal-poor stars are driven by individual enrichment events and the mode ls of Woosley & Weaver, the data for CS 22876-032 are consistent with its h aving 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 primordi al 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].