J0023+0307: A mega metal-poor dwarf star from SDSS/BOSS
  Only a handful of stars have been identified with an iron abundance
[Fe/H]<-5, and only one at [Fe/H]<-7. These stars have very large
carbon-to-iron ratios, with A(C)~7.0, most likely due to fallback in
core-collapse supernovae, which makes their total metallicity Z much higher
than their iron abundances. The failure to find population III stars, those
with no metals, has been interpreted, with support from theoretical modeling,
as the result of a top-heavy initial mass function. With zero or very low metal
abundance limiting radiative cooling, the formation of low-mass stars could be
inhibited. Currently, the star SDSS J1029+1729 sets the potential metallicity
threshold for the formation of low-mass stars at log Z/Z_0~-5. We have
identified SDSS J0023+0307, a primitive star with T_eff =6188+-84 K, and log
g=4.9+-0.5, an upper limit [Fe/H]< -6.6, and a carbon abundance A(C)< 6.3. In
our quest to push down the metallicity threshold we find J0023+0307 to be one
of the two most iron-poor stars known, and it exhibits less carbon that most of
stars at [Fe/H]< -5.
