A comparative study of three modes of s-process nucleosynthesis in extremely metal-poor AGB stars

Carbon-enhanced metal-poor (CEMP) stars in the Galactic halo have a wide range of neutron-capture element abundance patterns. To identify their origin, we investigate three modes of s-process nucleosynthesis that have been proposed to operate in extremely metal-poor asymptotic giant branch (AGB) stars: convective C-13 burning, which occurs when hydrogen is engulfed by helium-flash convection in low-mass AGB stars; convective Ne-22 burning, which occurs in helium-flash convection of intermediate-mass AGB stars; and radiative C-13 burning, which occurs in the C-13 pocket that is formed during inter-pulse periods. We show that the production of s-process elements per iron seed (s-process efficiency) does not depend on metallicity below [Fe//H] = -2, because O-16 in the helium zone dominates the neutron poison. The convective C-13 mode can produce a variety of s-process efficiencies for Sr, Ba, and Pb, including the maxima observed among CEMP stars. The Ne-22 mode only produces the lowest end of s-process efficiencies among CEMP models. We show that the combination of these two modes can explain the full range of observed enrichment of s-process elements in CEMP stars. In contrast, the C-13 pocket mode can hardly explain the high level of enrichment observed in some CEMP stars, even if considering star-to-star variations of the mass of the C-13 pocket. These results provide a basis for discussing the binary mass transfer origin of CEMP stars and their subgroups.