Heidelberg Joint Astronomical Colloquium

It has been recognized for some time that the halo system of the Galaxy exhibits a large fraction of metal-poor stars ([Fe/H] < -1.5) with significant overabundances of carbon, [C/Fe] > +0.7. These are known collectively as Carbon-Enhanced Metal-Poor (CEMP) stars. The great majority of these stars, roughly 80%, exhibit heavy element abundance patterns typically associated with the s-process (CEMP-s), and hence are thought to be produced by mass transfer from the envelope of a binary companion that has passed through the AGB stage. However, roughly 20% of the CEMP stars show no enhancements of neutron-capture elements. These so-called CEMP-no stars are also primarily found at lower metallicity, and dominate samples of halo CEMP stars with [Fe/H] < -2.5.

After summarizing the nature of the CEMP stars, I present an argument that the inner halo of our Galaxy is dominated by the CEMP-s variety of such stars, while the outer halo is dominated by the CEMP-no stars. This inference may provide insight into the origin of the two populations, and the nucleosynthetic production of carbon and other light elements in the earliest generations of stars, thus completing the link between local metal-poor stars and high-redshift Damped Lyman Alpha (DLA) systems.