Metal-Poor Stars Observed with the Magellan Telescope II. Discovery of
  Four Stars with [Fe/H] < -3.5
  We report on the discovery of seven low-metallicity stars selected from the
Hamburg/ESO Survey, six of which are extremely metal-poor ([Fe/H]<-3.0), with
four having [Fe/H]<-3.5. Chemical abundances or upper limits are derived for
these stars based on high-resolution (R~35,000) Magellan/MIKE spectroscopy, and
are in general agreement with those of other very and extremely metal-poor
stars reported in the literature. Accurate metallicities and abundance patterns
for stars in this metallicity range are of particular importance for studies of
the shape of the metallicity distribution function of the Milky Way's halo
system, in particular for probing the nature of its low-metallicity tail. In
addition, taking into account suggested evolutionary mixing effects, we find
that six of the program stars (with [Fe/H]<-3.35) possess atmospheres that were
likely originally enriched in carbon, relative to iron, during their
main-sequence phases. These stars do not exhibit over-abundances of their
s-process elements, and hence may be additional examples of the so-called
CEMP-no class of objects.
