Diffusion Processes in Peculiar a Stars

It is suggested that diffusion processes are responsible for most of the peculiar abundances observed in Ap stars. If it is assumed that the atmosphete 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 Oeff, log g they are observed to have). Radiation pressure leads to the overabundances of Mn, Sr, V, Zr, and the rare earths in the stars where they are observed. Silicon would be expected to be overabundant only if it has wide autoionization features. Phosphorus would be expected to be overabundant in star's with 0.5, but is observed to be overabundant in stars with Oeff 0.4. The tnagnetic fields observed in Ap 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.

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