Helium shell

PN nucleus, horizontal-branch and white-dwarf regions. The dotted line shows a schematic main sequence and evolutionary track for Population II, while various dashed lines show roughly the Cepheid instability strip, the transition to surface convection zones and the helium-shell flashing locus for Population I. After Pagel (1977). Copyright by the IAU. Reproduced with kind permission from Kluwer Academic Publishers. 

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. 

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 third dredge-up occurs as a result of the envelope convection extending downward in mass during a helium shell flash, with the result that the products of both the CNO cycle and the 3-a process are brought to the stellar surface (Iben 1975). As a result of the third dredge-up, the stellar surface is enriched in 12C and s-process elements. 

However, there are two sets of observations which indicate that AGB stars do indeed reach the AGB limit Mbol = -7.1. Firstly, the results of WBF and Reid, Glass and Catchpole (1987) for the LPVs show that such stars do reach the AGB limit. Secondly, a luminosity function for red stars in the bar of the LMC obtained by Hughes and Wood (1987) shows that AGB stars extend to Mbol = -7.1, although there is a steep fall-off with luminosity above Mboi = -6. One explanation for the rapid fall-off in the number of AGB stars above Mboi = -6 is provided by Wood and Faulkner (1986) who show that envelope ejection will occur at this point in all but the most massive AGB stars due to the luminosity of the star exceeding the Eddington limit at the base of the hydrogen-rich envelope during the surface luminosity peak of a helium shell flash. 

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. 

HASS EJECTION DURING HELIUM SHELL FLASHES FROM A MASSIVE WHITE DWARF... 

Abstract We have simulated the helium shell flashes on an 1.3 M0 white dwarf and estimated the amount of mass loss. Our results may suggest a serious difficulty for the theories of the formation of Type I supernovae and of the accretion-induced formation of neutron stars because a significant amount of envelope mass is ejected during a helium shell flash. 

Figure 1 The ratio of the lost mass to the accreted mass for each cycle of helium shell flashes as a function of the helium accretion rate.

Mass Ejection During Helium Shell Flashes from a Massive White Dwarf... 

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. 

The environment provided by thermal pulses in the helium shells of intermediate-mass stars on the AGB provides conditions consistent with the synthesis of the bulk of the heavy s-process isotopes through bismuth. Neutron captures in AGB stars are driven by a combination of neutron sources the C(a, n) 0 reaction provides... 

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