Hydrogen and helium burning in zero-metal asymptotic giant branch stars and the existence of thresholds (in core mass and CNO abundance) for the occurrence of helium shell flashes

Details Hydrogen and helium burning in zero-metal asymptotic giant branch stars and the existence of thresholds (in core mass and CNO abundance) for the occurrence of helium shell flashes Hydrogen and helium burning in zero-metal asymptotic giant branch stars and the existence of thresholds (in core mass and CNO abundance) for the occurrence of helium shell flashes

Dec 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984apj...287..749f&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 287, Dec. 15, 1984, p. 749-760.

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Computation

31

Asymptotic Giant Branch Stars, Giant Stars, Helium, Hydrogen, Nuclear Fusion, Stellar Evolution, Abundance, Carbon, Combustion Stability, Computational Astrophysics, Milky Way Galaxy, Nitrogen, Oxygen, Stellar Cores, Stellar Envelopes, Stellar Mass, Thermal Stability

Scientific paper

Using a semianalytical method, the properties of hydrogen and helium burning in shells above the electron- degenerate cores of asymptotic giant branch stars are studied as a function of core mass M1 and of the abundance Ze of CNO elements preexisting in the envelope. In zero-metal model stars (defined as stars in which Ze is zero), the temperature T1 in the hydrogen-burning shell is large enough that 3α reactions produce locally sufficient 12C for hydrogen burning by the CN cycle to dominate in energy production, provided M1 > 0.5 Msun. Because of the increase in T1 with increasing core mass, the stability of helium shell burning changes its nature at a core mass M1* ≈ 0.73 Msun. For M1 M1*, helium burning is stable and settles into a steady state. For M1 > M1*, there exists a critical envelope CNO abundance Ze* such that, if Ze > Ze*, helium shell flashes will be triggered. For M1 in the range (0.8-1.4) Msun, Ze* increases exponentially with M1 from 10-7 to 10-4 . The evolution of zero-metal stars and their contribution to Galactic nucleosynthesis are discussed. For stars with a small total mass (<4 Msun), the initial core mass at the onset of double shell burning is smaller than M1*, and helium shell flashes occur. Since the CNO abundance in the envelope increases as a result of a dredge-up process which operates for M1 > 0.6 Msun, helium shell flashes will continue to occur even after M1 grows larger than M1*. Such stars may therefore contribute substantially to the production of 12C and neutron-rich isotopes of the light elements. Stars with an initial mass larger than 4 Msun enter the AGB stage with M1 > M1*, and never experience helium shell flashes. They may become supernovae and contribute substantially to the production of Ni and Fe.

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