Oxygen and light-element synthesis by neutron-capture reactions in metal-free and extremely metal-poor AGB stars

The metal-free (Population III: Pop. III) and extremely metal-poor (EMP) stars of low-and intermediate-masses experience mixing of hydrogen into the helium convection during the early TP-AGB phase, differently from the metal-rich stars. We have studied nucleosynthesis in the helium convective zone with 13C formed from mixed protons as a neutron source by using a nuclear network from H through S. In the absence or scarcity of pristine metals, neutron-recycling reactions, l2C (n, γ) 13C (α, n) 16O and also 16O (n, γ) 17O (α, n) 20Ne promote the synthesis of O and light elements, including their neutron-rich isotopes and the odd atomic number elements. Based on the results, we demonstrate that the peculiar abundance patterns of C through Al observed for the three most irondeficient, carbon-rich stars can be reproduced in terms of nucleosynthesis in Pop. III, AGB stars in three different mass ranges. We argue that these three stars were born as low-mass members of Pop. III binaries, and later were subjected to surface pollution by mass transfer in the binary systems. It is also shown that AGB nucleosynthesis with hydrogen mixing explains the abundances of C, O, Na, Mg, and Al observed for most of carbon-enhanced EMP (CEMP) stars, including all CEMP-s stars with s-process elements. In addition, the present results are used to single out nucleosynthetic signatures of the early generations of stars other than AGB stars. © 2009. Astronomical Society of Japan.SCOPUS: ar.j