A Unique Star in the Outer Halo of the Milky Way
  As part of a program to measure abundance ratios in stars beyond 15 kpc from
the Galactic center, we have discovered a metal-poor star in the outer halo
with a unique chemical signature. We originally identified it in the Sloan
Extension for Galactic Understanding and Exploration (SEGUE) survey as a
distant metal-poor star. We obtained a follow-up spectrum using the Echellette
Spectrograph and Imager (ESI) at the Keck 2 telescope, and measure [Fe/H]
=-3.17, [Mg/Fe] =-0.10 and [Ca/Fe] =+1.11. This is one of the largest
over-abundances of Ca measured in any star to date; the extremely low value of
[Mg/Ca]=-1.21 is entirely unique. To have found such an unusual star in our
small sample of 27 targets suggests that there may be previously unobserved
classes of stars yet to be found in situ in the Galactic halo.
