The origin of carbon enhancement and the initial mass function of extremely metal-poor stars in the galactic halo

It is known that the carbon-enhanced, extremely metal-poor (CEMP) stars constitute a substantial proportion of the extremely metal-poor (EMP) stars of the Galactic halo, and a by far larger proportion than CH stars among Population II stars. We investigate their origin by taking into account an additional evolutionary path to the surface carbon enrichment, triggered by hydrogen engulfment by the helium flash convection, in EMP stars with [Fe/H] less than or similar to -2.5. This process is distinct from the third dredge-up operating in more metal-rich stars and in EMP stars. In binary systems of EMP stars, the secondary stars become CEMP stars through mass transfer from the low-and intermediate-mass primary stars that have developed the surface carbon enhancement. Our binary scenario can predict the variations in the abundances not only for carbon but also for nitrogen and s-process elements and can reasonably explain the observed properties such as the stellar distributions of the carbon abundances, the binary periods, and the evolutionary stages. Furthermore, from the observed frequencies of CEMP stars with and without s-process element enhancement, we demonstrate that the initial mass function of EMP stars needed gives the mean mass similar to 10 M-circle dot under the reasonable assumptions for the distributions of orbital separations and mass ratios of the binary components. This also indicates that the currently observed EMP stars were exclusively born as the secondary members of binaries, making up similar to 10% of EMP binary systems, with mass similar to 10(8) M-circle dot in total; in addition to CEMP stars with white dwarf companions, a significant fraction of them have experienced supernova explosions of their companions. We discuss the implications of the present results for the formation of the Galactic halo.