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 ( SDSS ) and the Sloan Extension for Galactic Understanding and Exploration ( SEGUE ) , 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 M _ { \odot } .
Previous observations suggest that the carbon abundances of red giants are altered during red-giant branch evolution due to mixing of their convective outer layers , resulting in a reduction of the observed carbon-abundance ratios .
Thus , in order to derive more accurate estimates of CEMP frequencies for stars in the Milky Way , it is preferable to make use of SDSS/SEGUE main-sequence turnoff stars , which are not expected to experience significant dilution .
However , because of the difficulty of identifying moderately carbon-enhanced stars ( +0.7 < [ C/Fe ] < +1.5 ) among warm , metal-poor turnoff stars , owing to their much weaker CH G -bands , we derive a correction function to compensate for the resulting undercounts of CEMP stars .
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 .