Hydrogen shell burning

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,... 

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. 

A small percentage of the27 A1 is created instead in the hydrogen-burning shells of evolved stars, where the 2 Mg + 2H —27 A1 reaction can occur. At the time of the supernova explosion, this shell burning can have almost doubled the27A1 abundance locally in the hydrogen convective shell that has been exposed at the stellar surface. 

Second, the shape of the evolution track is altered. In a star with a radiative core nuclear burning shifts smoothly from the hydrogen-depleted core to a thick hydrogen-poor shell and then to a thin hydrogen-rich shell. The transition from main-sequence to post main-sequence evolution is comparatively smooth (Fig. 8). In a star with a convective core, nuclear burning switches off abruptly as the entire convective core is exhausted. This precipitates a minor collapse of the depleted core before ignition of the hydrogen shell, and produces a hook in the evolution track at the end of main-sequence evolution. 

It is on the TP-AGB that the richest nucleosynthesis occurs for low and intermediate-mass stars, even though stars spend such a short amount of time there compared to previous evolutionary phases. The nucleosynthesis is driven by thermal instabilities of the helium-burning shell, reviewed in Sect. 4. Of particular importance is the action of repeated third dredge-up events that mix the products of He-burning to the stellar surface. Material from the He-shell will become part of the next hydrogen shell, where they will experience proton captures during the next interpulse period. For this reason, not only do we need to consider nucleosynthesis in the thermal pulse itself but also... 

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