Nuclear and non-nuclear processes in the production of peculiar A stars

Recent theories of the origin of the abundance anomalies on Ap stars are reviewed. The abundance anomalies are mainly the results of supernova explosions of companion stars and magnetic accretion from the supernova remnants and the interstellar medium. Large rare-earth excesses indicate that magnetic accretion has taken place, while large excesses of Pt and Hg are due to the transfer of material from nearby supernovae. Nuclear reactions involving protons may also have taken place around Ap stars. There are several objections to Michaud's diffusion theory as a comprehensive explanation for Ap abundance anomalies.