The extremely low-metallicity tail of the Sculptor dwarf spheroidal
  galaxy
  We present abundances for seven stars in the (extremely) low-metallicity tail
of the Sculptor dwarf spheroidal galaxy, from spectra taken with X-shooter on
the ESO VLT. Targets were selected from the Ca II triplet (CaT) survey of the
Dwarf Abundances and Radial Velocities Team (DART) using the latest
calibration. Of the seven extremely metal-poor candidates, five stars are
confirmed to be extremely metal-poor (i.e., [Fe/H]<-3 dex), with [Fe/H]=-3.47
+/- 0.07 for our most metal-poor star. All are around or below [Fe/H]=-2.5 dex
from the measurement of individual Fe lines. These values are in agreement with
the CaT predictions to within error bars. None of the seven stars is found to
be carbon-rich. We estimate a 2-13% possibility of this being a pure chance
effect, which could indicate a lower fraction of carbon-rich extremely
metal-poor stars in Sculptor compared to the Milky Way halo. The [alpha/Fe]
ratios show a range from +0.5 to -0.5, a larger variation than seen in Galactic
samples although typically consistent within 1-2sigma. One star seems mildly
iron-enhanced. Our program stars show no deviations from the Galactic abundance
trends in chromium and the heavy elements barium and strontium. Sodium
abundances are, however, below the Galactic values for several stars. Overall,
we conclude that the CaT lines are a successful metallicity indicator down to
the extremely metal-poor regime and that the extremely metal-poor stars in the
Sculptor dwarf galaxy are chemically more similar to their Milky Way halo
equivalents than the more metal-rich population of stars.
