1999A&A...351..554H

Radiatively driven winds occur in all main sequence stars (Babel, 1995A&A...301..823B, 1996A&A...309..867B). However, due to the weak coupling between the photon absorbing metals and the inert elements H and He, the wind in the low temperature domain is fractionated: He decouples from the wind at T_eff<25000K, and below 17000K even H. The decoupled elements fall back to the surface of the star thus creating overabundances and abundance stratifications. These anomalies, however, become manifest only if atmospheric turbulence is suppressed (say by magnetic fields). In order to prove the validity of the described scenario, all B_p stars for which reliable fundamental parameters exist, are discussed on the basis of the (augmented) (g, T_eff)-diagram of Babel (1996A&A...309..867B). It is shown that the fractionation process is able to explain the observed temperature sequence of He-rich and He-poor stars, additionally to classical diffusion processes. A necessary condition is that a magnetic field is present. This explains why only a fraction of B stars exhibits H/He anomalies. While classical diffusion operates in the quiet zones (no wind) of a star, fractionation takes place above the wind bases.