Carbon-enhanced Metal-poor Stars in SDSS/SEGUE. II. Comparison of CEMP
  Star Frequencies with Binary Population Synthesis Models
  We present a comparison of the frequencies of carbon-enhanced metal-poor
(CEMP) giant and main-sequence turnoff stars, selected from the Sloan Digital
Sky Survey and the Sloan Extension for Galactic Understanding and Exploration,
with predictions from asymptotic giant-branch (AGB) mass-transfer models. We
consider two initial mass functions (IMFs)-a Salpeter IMF, and a mass function
with a characteristic mass of 10 solar mass. These comparisons indicate good
agreement between the observed CEMP frequencies for stars with [Fe/H] > -1.5
and a Salpeter IMF, but not with an IMF having a higher characteristic mass.
Thus, while the adopted AGB model works well for low-mass progenitor stars, it
does not do so for high-mass progenitors. Our results imply that the IMF
shifted from high- to low-mass dominated in the early history of the Milky Way,
which appears to have occurred at a "chemical time" between [Fe/H] = -2.5 and
[Fe/H] = -1.5. The corrected CEMP frequency for the turnoff stars with [Fe/H] <
-3.0 is much higher than the AGB model prediction from the high-mass IMF,
supporting the previous assertion that one or more additional mechanisms, not
associated with AGB stars, are required for the production of carbon-rich
material below [Fe/H] = -3.0. [abridged]
