A few rare halo giants in the range [ Fe/H ] \simeq - 2.9 \pm 0.3 exhibit r -process element abundances that vary as a group by factors up to [ r /Fe ] \sim 80 , relative to those of the iron peak and below .
Yet , the astrophysical production site of these r -process elements remains unclear .
We report initial results from four years of monitoring the radial velocities of 17 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 s -process elements . We conclude that the 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 .