Asymptotic giant branch stars as producers of carbon and of neutron-rich isotopes

  • Icko IbenJr.
5. Post-Main Sequence Stars
Part of the Lecture Notes in Physics book series (LNP, volume 193)


Carbon stars are thought to be in the asymptotic giant branch (AGR) phase of evolution, alternately burning hydrogen and helium in shells above an electronde-generate carbon-oxygen (CO) core. The excess of carbon relative to oxygen at the surfaces of these stars is thought to be due to convective dredge-up which occurs following a thermal pulse. During a thermal pulse, carbon and neutron-rich isotopes are made in a convective helium-burning zone. In model stars of large CO core mass, the source of neutrons for producing the neutron-rich isotopes is the 22Ne(α,n)25Mg reaction and the isotopes are produced in the solar system s-process distribution. In models of small core mass, the 13C(α,n)16O reaction is thought to be responsible for the release of neutrons, and the resultant distribution of neutron-rich isotopes is expected to vary considerably from one star to the next, with the distribution in isolated instances possibly resembling the solar system distribution of r-process isotopes. After the dredge-up phase following each pulse, the 13C is made by the reactions 12C(P,γ)13N(β+ν)13C in a zone of large 12C abundance and small 1H abundance that has been established by semiconvective mixing during the dredge-up phase. There is qualitative accord between the properties of carbon stars in the Magellanic Clouds and properties of model stars, but considerably more theoretical work is required before a quantitative match is achieved.


Globular Cluster Core Mass Planetary Nebula Model Star Asymptotic Giant Branch 
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Copyright information

© Springer-Verlag 1984

Authors and Affiliations

  • Icko IbenJr.
    • 1
  1. 1.Department of AstronomyUniversity of Illinois 341 Astronomy BuildingUrbanaUSA

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