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We discuss the carbon-normal and carbon-rich populations of Galactic halostars having [Fe/H] < -3.0, utilizing chemical abundances from high-resolution,high-S/N model-atmosphere analyses. The C-rich population represents ~28% ofstars below [Fe/H] = -3.1, with the present C-rich sample comprising 16 CEMP-nostars, and two others with [Fe/H] ~ -5.5 and uncertain classification. Thepopulation is O-rich ([O/Fe] > +1.5); the light elements Na, Mg, and Al areenhanced relative to Fe in half the sample; and for Z > 20 (Ca) there is littleevidence for enhancements relative to solar values. These results are bestexplained in terms of the admixing and processing of material from H-burningand He-burning regions as achieved by nucleosynthesis in zero-heavy-elementmodels in the literature of "mixing and fallback" supernovae (SNe); ofrotating, massive and intermediate mass stars; and of Type II SNe withrelativistic jets. The available (limited) radial velocities offer littlesupport for the C-rich stars with [Fe/H] < -3.1 being binary. More data arerequired before one could conclude that binarity is key to an understanding ofthis population. We suggest that the C-rich and C-normal populations resultfrom two different gas cooling channels in the very early Universe, of materialthat formed the progenitors of the two populations. The first was cooling byfine-structure line transitions of CII and OI (to form the C-rich population);the second, while not well-defined (perhaps dust-induced cooling?), led to theC-normal group. In this scenario, the C-rich population contains the oldeststars currently observed.

The Most Metal-Poor Stars. IV. The Two Populations With [Fe/H] < -3.0