  Very metal-deficient stars that exhibit enhancements of their carbon
abundances are of crucial importance for understanding a number of issues --
the nature of stellar evolution among the first generations of stars, the shape
of the Initial Mass Function, and the relationship between carbon enhancement
and neutron-capture processes, in particular the astrophysical s-process. One
recent discovery from objective-prism surveys dedicated to the discovery of
metal-deficient stars is that the frequency of Carbon-Enhanced Metal-Poor
(CEMP) stars increases with declining metallicity, reaching roughly 25% for
[Fe/H] < -2.5. In order to explore this phenomenon in greater detail we have
obtained medium-resolution (2 A) spectroscopy for about 350 of the 413 objects
in the Christlieb et al. catalog of carbon-rich stars, selected from the
Hamburg/ESO objective prism survey on the basis of their carbon-enhancement,
rather than metal deficiency. Based on these spectra, and near-IR JHK
photometry from the 2MASS Point Source Catalog, we obtain estimates of [Fe/H]
and [C/Fe] for most of the stars in this sample, along with reasonably accurate
determinations of their radial velocities. Of particular importance, we find
that the upper envelope of carbon enhancement observed for these stars is
nearly constant, at [C/H] ~ -1.0, over the metallicity range -4.0 < [Fe/H] <
-2.0; this same level of [C/H] applies to the most iron-deficent star yet
discovered, HE 0107-5240, at [Fe/H] = -5.3.

Astrophysics