The NIR Ca II triplet at low metallicity - Searching for extremely
  low-metallicity stars in classical dwarf galaxies
  The NIR Ca II triplet absorption lines have proven to be an important tool
for quantitative spectroscopy of individual red giant branch stars in the Local
Group, providing a better understanding of metallicities of stars in the Milky
Way and dwarf galaxies and thereby an opportunity to constrain their chemical
evolution processes. An interesting puzzle in this field is the significant
lack of extremely metal-poor stars, below [Fe/H]=-3, found in classical dwarf
galaxies around the Milky Way using this technique. The question arises whether
these stars are really absent, or if the empirical Ca II triplet method used to
study these systems is biased in the low-metallicity regime. Here we present
results of synthetic spectral analysis of the Ca II triplet, that is focused on
a better understanding of spectroscopic measurements of low-metallicity giant
stars. Our results start to deviate strongly from the widely-used and linear
empirical calibrations at [Fe/H]<-2. We provide a new calibration for Ca II
triplet studies which is valid for -0.5<[Fe/H]<-4. We subsequently apply this
new calibration to current data sets and suggest that the classical dwarf
galaxies are not so devoid of extremely low-metallicity stars as was previously
thought.
