Population Synthesis of Binary Carbon-enhanced Metal-poor Stars
  The carbon-enhanced metal-poor (CEMP) stars constitute approximately one
fifth of the metal-poor ([Fe/H] ~< -2) population but their origin is not well
understood. The most widely accepted formation scenario, invokes mass-transfer
of carbon-rich material from a thermally-pulsing asymptotic giant branch
(TPAGB) primary star to a less massive main-sequence companion which is seen
today. Recent studies explore the possibility that an initial mass function
biased toward intermediate-mass stars is required to reproduce the observed
CEMP fraction in stars with metallicity [Fe/H] < -2.5. These models also
implicitly predict a large number of nitrogen-enhanced metal-poor (NEMP) stars
which is not seen. We investigate whether the observed CEMP and NEMP to
extremely metal-poor (EMP) ratios can be explained without invoking a change in
the initial mass function.
  We confirm earlier findings that with current detailed TPAGB models the large
observed CEMP fraction cannot be accounted for. We find that efficient third
dredge up in low-mass (less than 1.25Msun), low-metallicity stars may offer at
least a partial explanation to the large observed CEMP fraction while remaining
consistent with the small observed NEMP fraction.
