The most metal-poor stars. III. The metallicity distribution function and carbon-enhanced metal-poor fraction

• We examine the metallicity distribution function (MDF) and fraction of carbon-enhanced metal-poor (CEMP) stars in a sample that includes 86 stars with [Fe/H] <= -3.0, based on high-resolution, high signal-to-noise spectroscopy, of which some 32 objects lie below [Fe/H] = -3.5. After accounting for the completeness function, the "corrected" MDF does not exhibit the sudden drop at [Fe/H] = -3.6 that was found in recent samples of dwarfs and giants from the Hamburg/ESO survey. Rather, the MDF decreases smoothly down to [Fe/H] = -4.1. Similar results are obtained from the "raw" MDF. We find that the fraction of CEMP objects below [Fe/H] = -3.0 is 23% +/- 6% and 32% +/- 8% when adopting the Beers & Christlieb and Aoki et al. CEMP definitions, respectively. The former value is in fair agreement with some previous measurements, which adopt the Beers & Christlieb criterion.

Nyckelord

early universe

Galaxy: formation

Galaxy: halo

nuclear reactions

nucleosynthesis

abundances

stars: abundances

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