Helium shell burning

Electron-density effects on fi-decay lifetimes also enable the total density to be placed in the range 2500 to 13 000 gm cm-3 all these parameters are characteristic of helium shell-burning zones as expected. 

In this review we wish to discuss how observations of AGB stars can be used to determine the manner in which heavy elements are created during a thermal pulse, and how these heavy elements and carbon are transported to the stellar surface. In particular we wish to study how the periodic hydrogen and helium shell burning above a degenerate carbon-oxygen (C-0) core forms a neutron capture nucleosynthesis site that may eventually account for the observed abundance enhancements at the surfaces of AGB stars. In section II we discuss the nucleosynthesis provided by stellar evolution models (for a general review see [1]). In section III we discuss the isotopic abundances provided by nucleosynthesis reaction network calculations (see [2, 3]). In section IV we discuss how observations of AGB stars can be used to discriminate between the neutron capture nucleosynthesis sources (see [4]). And in section V we note some of the current uncertainty in this work. 

The helium shell burning instability has two consequences. As Fig. 18 implies, not only the nuclear luminosity (JM enucdM) becomes large at the peak of the instability, but also the surface luminosity. Quickly, values exceeding the white dwarf s Eddington luminosity are achieved. This will lead to radiation driven mass loss from the white dwarf (cf. Kato Hachisu 1999), which questions whether it can ever reach the Chandrasekhar limit. 

Helium flash A rapid burst of nuclear reactions in the hydrogen-shell burning phase of stellar evolution. 

The s-process occurs during the AGB stage in low and intermediate mass stars, when the hydrogen shell is burning outward from the core and the helium shell repeatedly ignites,... 

The advantage of the helium shells of low- and intermediate-mass stars for production arises from the fact that, for conditions of incomplete helium burning, the ratio is high. 

Following core-helium exhaustion, there being no H-burning shell, the mass of the helium-rich intershell remains fixed. A few mild helium-shell ignitions may occur11, but the star is ultimately doomed not to reach the asymptotic giant branch. Instead it will contract directly to become a hybrid He/CO white dwarf. 

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