Observational Constraints on First-Star Nucleosynthesis. I. Evidence for
  Multiple Progenitors of CEMP-no Stars
  We investigate anew the distribution of absolute carbon abundance, $A$(C) $=
\log\,\epsilon $(C), for carbon-enhanced metal-poor (CEMP) stars in the halo of
the Milky Way, based on high-resolution spectroscopic data for a total sample
of 305 CEMP stars. The sample includes 147 CEMP-$s$ (and CEMP-r/s) stars, 127
CEMP-no stars, and 31 CEMP stars that are unclassified, based on the currently
employed [Ba/Fe] criterion. We confirm previous claims that the distribution of
$A$(C) for CEMP stars is (at least) bimodal, with newly determined peaks
centered on $A$(C)$=7.96$ (the high-C region) and $A$(C)$ =6.28$ (the low-C
region). A very high fraction of CEMP-$s$ (and CEMP-r/s) stars belong to the
high-C region, while the great majority of CEMP-no stars reside in the low-C
region. However, there exists complexity in the morphology of the $A$(C)-[Fe/H]
space for the CEMP-no stars, a first indication that more than one class of
first-generation stellar progenitors may be required to account for their
observed abundances. The two groups of CEMP-no stars we identify exhibit
clearly different locations in the $A$(Na)-$A$(C) and $A$(Mg)-$A$(C) spaces,
also suggesting multiple progenitors. The clear distinction in $A$(C) between
the CEMP-$s$ (and CEMP-$r/s$) stars and the CEMP-no stars appears to be $as\
successful$, and $likely\ more\ astrophysically\ fundamental$, for the
separation of these sub-classes as the previously recommended criterion based
on [Ba/Fe] (and [Ba/Eu]) abundance ratios. This result opens the window for its
application to present and future large-scale low- and medium-resolution
spectroscopic surveys.
