the lowest metallicities could indicate that the IMF in the early universe included a larger number of intermediate- to high-mass stars than the present-day IMF. However, Izzard et al. (2009) and Abate et al. (2013), using a binary population model alone, could not reproduce the high fraction of CEMP stars among the most metal-poor stars.

In order to understand the carbon-enhanced phenomenon among low-metallicity stars, there have been a number of efforts to estimate carbon abundance from low- and medium-resolution stellar spectroscopic surveys of metal-poor candidates. Generally, these efforts were limited in terms of sample size, coverage of stellar parameter space, and methodology for determining [\[C/Fe\]]{}. For example, the early approach of Rossi et al. (2005) made use of the strength of the Ca[ii]{} K line and CH $G$-band, along with the associated broadband $J-K$ colors, to estimate \[Fe/H\] and \[C/Fe\] for medium-resolution (1–2 Å) spectra of HK-survey stars. Frebel et al. (2006) followed Rossi et al.’s prescription to obtain [\[C/Fe\]]{} for a subset of 234 stars among 1777 metal-poor candidates from the HES survey, deriving a frequency of 9%$\pm$2% for VMP giants with [\[C/Fe\]]{} $> +1.0$. Very recently, Carollo et al. (2012) employed a grid of synthetic spectra, covering 