  We report the discovery of a cool metal-poor, main-sequence star exhibiting
large excesses of r-process elements. This star is one of two newly discovered
cool subdwarfs (effective temperatures of 5000 K) with extremely low
metallicity ([Fe/H]<-3) identified from follow-up high-resolution spectroscopy
of metal-poor candidates from the Sloan Digital Sky Survey. SDSS J2357-0052 has
[Fe/H]=-3.4 and [Eu/Fe]=+1.9, and exhibits a scaled solar r-process abundance
pattern of heavy neutron-capture elements. This is the first example of an
extremely metal-poor, main-sequence star showing large excesses of r-process
elements; all previous examples of the large r-process-enhancement phenomena
have been associated with metal-poor giants. The metallicity of this object is
the lowest, and the excess of Eu ([Eu/Fe]) is the highest, among the
r-process-enhanced stars found so far. We consider possible scenarios to
account for the detection of such a star, and discuss techniques to enable
searches for similar stars in the future.
