  Recent studies suggest that metal-poor stars enhanced in carbon but
containing low levels of neutron-capture elements may have been among the first
to incorporate the nucleosynthesis products of the first generation of stars.
We have observed 16 stars with enhanced carbon or nitrogen using the MIKE
Spectrograph on the Magellan Telescopes at Las Campanas Observatory and the
Tull Spectrograph on the Smith Telescope at McDonald Observatory. We present
radial velocities, stellar parameters, and detailed abundance patterns for
these stars. Strontium, yttrium, zirconium, barium, europium, ytterbium, and
other heavy elements are detected. In four stars, these heavy elements appear
to have originated in some form of r-process nucleosynthesis. In one star, a
partial s-process origin is possible. The origin of the heavy elements in the
rest of the sample cannot be determined unambiguously. The presence of elements
heavier than the iron group offers further evidence that zero-metallicity
rapidly-rotating massive stars and pair instability supernovae did not
contribute substantial amounts of neutron-capture elements to the regions where
the stars in our sample formed. If the carbon- or nitrogen-enhanced metal-poor
stars with low levels of neutron-capture elements were enriched by products of
zero-metallicity supernovae only, then the presence of these heavy elements
indicates that at least one form of neutron-capture reaction operated in some
of the first stars.
