The Most Metal-Poor Stars. IV. The Two Populations With [Fe/H] < -3.0
  We discuss the carbon-normal and carbon-rich populations of Galactic halo
stars having [Fe/H] < -3.0, utilizing chemical abundances from high-resolution,
high-S/N model-atmosphere analyses. The C-rich population represents ~28% of
stars below [Fe/H] = -3.1, with the present C-rich sample comprising 16 CEMP-no
stars, and two others with [Fe/H] ~ -5.5 and uncertain classification. The
population is O-rich ([O/Fe] > +1.5); the light elements Na, Mg, and Al are
enhanced relative to Fe in half the sample; and for Z > 20 (Ca) there is little
evidence for enhancements relative to solar values. These results are best
explained in terms of the admixing and processing of material from H-burning
and He-burning regions as achieved by nucleosynthesis in zero-heavy-element
models in the literature of "mixing and fallback" supernovae (SNe); of
rotating, massive and intermediate mass stars; and of Type II SNe with
relativistic jets. The available (limited) radial velocities offer little
support for the C-rich stars with [Fe/H] < -3.1 being binary. More data are
required before one could conclude that binarity is key to an understanding of
this population. We suggest that the C-rich and C-normal populations result
from two different gas cooling channels in the very early Universe, of material
that formed the progenitors of the two populations. The first was cooling by
fine-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 the
C-normal group. In this scenario, the C-rich population contains the oldest
stars currently observed.
