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Paper: The Evolution of Metal-Deficient Stars and the Search for Population III Stars
Volume: 165, The Third Stromlo Symposium: The Galactic Halo
Page: 243
Authors: Fujimoto, Masayuki Y.; Aikawa, Masayuki; Iben, Icko, Jr.
Abstract: The most unique aspect of the evolution of extremely metal-deficient stars is the mixing and nucleosynthesis, triggered by the erosion of hydrogen-rich material into the helium convection. It occurs during the helium shell flashes and results in the formation of a carbon star, enriched with C and N. This differs from the third dredge-up in the Populations I and II AGB stars, which enriches C. The stars of mass M <= 1.1 msun and of metallicity [Fe / H] < -4 undergo this episode at the tip of red-giant branch when helium first ignites in the core. The rest of stars with M <= 3 msun and [Fe / H] < -2 postpone it until the helium shell flashes start at the base of AGB. More massive stars suffer from the surface abundance changes neither due to the hydrogen mixing nor to the third dredge-up for sufficiently low metallicity. The first generation stars that survive today show up as CN rich stars on the horizontal and asymptotic giant branches. Those in the binary systems may also be imprinted with the CN enhancement through the mass transfer from the companions. The Population III stars are most likely to be found among metal-deficient carbon stars.
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