The role of binaries in the enrichment of the early Galactic halo. III.
  Carbon-enhanced metal-poor stars -- CEMP-s stars
  Detailed spectroscopic studies of metal-poor halo stars have highlighted the
important role of carbon-enhanced metal-poor (CEMP) stars in understanding the
early production and ejection of carbon in the Galaxy and in identifying the
progenitors of the CEMP stars among the first stars formed after the Big Bang.
Recent work has also classified the CEMP stars by absolute carbon abundance,
A(C), into high- and low-C bands, mostly populated by binary and single stars,
respectively. Our aim is to determine the frequency and orbital parameters of
binary systems among the CEMP-s stars, which exhibit strong enhancements of
neutron-capture elements associated with the s-process. This allows us to test
whether local mass transfer from a binary companion is necessary and sufficient
to explain their dramatic carbon excesses. Eighteen of the 22 stars exhibit
clear orbital motion, yielding a binary frequency of 82+-10%, while four stars
appear to be single (18+-10%). We thus confirm that the binary frequency of
CEMP-s stars is much higher than for normal metal-poor giants, but not 100% as
previously claimed. Secure orbits are determined for 11 of the binaries and
provisional orbits for six long-period systems (P > 3,000 days), and orbital
circularisation time scales are discussed. The conventional scenario of local
mass transfer from a former AGB binary companion does appear to account for the
chemical composition of most CEMP-s stars. However, the excess of C and
s-process elements in some single CEMP-s stars was apparently transferred to
their natal clouds by an external (distant) source. This finding has important
implications for our understanding of carbon enrichment in the early Galactic
halo and some high-redshift DLA systems, and of the mass loss from extremely
metal-poor AGB stars. Abridged.
