2023PASJ...75.1145Y

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 ¹³C burning, which occurs when hydrogen is engulfed by helium-flash convection in low-mass AGB stars; convective ²²Ne burning, which occurs in helium-flash convection of intermediate-mass AGB stars; and radiative ¹³C burning, which occurs in the ¹³C 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 ¹⁶O in the helium zone dominates the neutron poison. The convective ¹³C mode can produce a variety of s-process efficiencies for Sr, Ba, and Pb, including the maxima observed among CEMP stars. The ²²Ne 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 ¹³C 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 ¹³C pocket. These results provide a basis for discussing the binary mass transfer origin of CEMP stars and their subgroups.