I. Diffusion processes in A-PEC stars. II. Nucleosynthesis in Si burning

Citation

Michaud, Georges (1970) I. Diffusion processes in A-PEC stars. II. Nucleosynthesis in Si burning. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ZR0T-W227. https://resolver.caltech.edu/CaltechETD:etd-05212008-112244

Abstract

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Two problems related to elemental abundances have been studied: the abundance anomalies in A-pec stars and the nucleosynthesis of elements between [...] and [...].

Diffusion processes were found to lead to most of the peculiar abundances observed in A-pec stars. If it is assumed that the atmos phere is stable enough for diffusion processes to be important, gravitational settling leads to the underabundances of He Ne and 0 in the stars where they are observed (that is with the [...] log g they are observed to have). Radiation pressure leads to the overabundances of Mn, Sr, Y, Zr and the rare earths in the stars where they are observed. Si would be expected to be overabundant onlyif it has wide auto-ionization features. The overabundance of P is not explained.

The magnetic fields observed in A-pec stars could bring to the atmosphere the stability needed for diffusion processes to be important. They would also guide diffusion into patches leading to the periodic variation of the observed overabundances.

We have also studied the conditions necessary to generate through nuclear reactions, the abundances observed between [...] and [...]. It was found that most of the observed abundances could have been generated in a single process, Si burning.

A mixture of [...] at t = O, in proportions leading to the observed abundance of [...] and [...] was found to [...] lead to the generation of most elements between Si and Co if one added up the zone where no Si had burned, where little had burned ... and where all had burned.

The summation over the zones was seen to depend on essentially two parameters which most conveniently can be expressed as the lower and upper limits of integration. One then has essentially four parameters: [...], and the two limits of integration. The density was found to have no effect on our results. The temperature is more important but the range of freezing temperatures to be expected can be estimated accurately enough from the dynamic time scale. With four free parameters, one than fits the abundances of some 25 to 30 of the 41 nuclei between [...] and [...]. Some of the poorly fitted nuclei are very neutron rich nuclei [...] which could easily have been formed by a slight exposure of the material to a neutron flux.

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