Diffusion in main-sequence stars - Radiation forces, time scales, anomalies

The abundance anomalies generated by diffusion in the envelope of main-sequence stars are studied. It is shown that in low-mass stars (not less than 1.2 solar masses) diffusion leads to underabundances while in more massive stars (not less than 1.3 solar masses) diffusion leads to overabundances of at least some elements. In general the overabundance and underabundance factors generated (up to 10 to the 7th) are larger than the observed anomalies in stars of the main sequence (rarely up to 10 to the 6th). It is established that diffusion can lead to the largest anomalies observed. For particular elements (Sr, Eu, . . .), it is shown where more accurate calculations are needed. Approximate formulae are developed for radiative accelerations.