The Pristine Survey IV: Approaching the Galactic metallicity floor with
  the discovery of an ultra metal-poor star
  The early Universe presented a star formation environment that was almost
devoid of heavy elements. The lowest metallicity stars thus provide a unique
window into the earliest Galactic stages, but are exceedingly rare and
difficult to find. Here we present the discovery of an ultra-metal-poor star,
Pristine_221.8781+9.7844, using narrow-band Ca H&K photometry from the Pristine
survey. Follow-up medium and high-resolution spectroscopy confirms the
ultra-metal-poor nature of Pristine_221.8781+9.7844 ([Fe/H] = -4.66 +/- 0.13 in
1D LTE) with an enhancement of 0.3-0.4 dex in alpha-elements relative to Fe,
and an unusually low carbon abundance. We derive an upper limit of A(C) = 5.6,
well below typical A(C) values for such ultra metal-poor stars. This makes
Pristine_221.8781+9.7844 one of the most metal-poor stars; in fact, it is very
similar to the most metal-poor star known (SDSS J102915+172927). The existence
of a class of ultra metal-poor stars with low(er) carbon abundances suggest
that there must have been several formation channels in the early Universe
through which long-lived, low-mass stars were formed.
