The Binary Frequency of r-Process-Element Enhanced Metal-Poor Stars and
  its Implications: Chemical Tagging in the Primitive Halo of the Milky Way
  A few rare halo giants in the range [Fe/H] $\simeq -2.9\pm0.3$ exhibit {\it
r}-process element abundances that vary as a group by factors up to [$r$/Fe]
$\sim80$, relative to those of the iron peak and below. Yet, the astrophysical
production site of these {\it r}-process elements remains unclear. We report
initial results from four years of monitoring the radial velocities of 17 {\it
r}-process-enhanced metal-poor giants to detect and characterise binaries in
this sample. We find three (possibly four) spectroscopic binaries with orbital
periods and eccentricities that are indistinguishable from those of Population
I binaries with giant primaries, and which exhibit no signs that the secondary
components have passed through the AGB stage of evolution or exploded as
supernovae. The other 14 stars in our sample appear to be single -- including
the prototypical $r$-process-element enhanced star CS 22892-052, which is also
enhanced in carbon, but not in {\it s}-process elements. We conclude that the
{\it r}-process (and potentially carbon) enhancement of these stars was not a
local event due to mass transfer or winds from a binary companion, but was
imprinted on the natal molecular clouds of these (single and binary) stars by
an external source. These stars are thus spectacular chemical tracers of the
inhomogeneous nature of the early Galactic halo system.
