Abstract
Aims: The effects of atomic diffusion on internal and surface abundances of A and F pre-main-sequence stars with mass loss are studied in order to determine at what age the effects materialize, as well as to further understand the processes at play in HAeBe and young ApBp stars. Methods: Self-consistent stellar evolution models of 1.5 to 2.8 Msun with atomic diffusion (including radiative accelerations) for all species within the OPAL opacity database were computed and compared to observations of HAeBe stars. Results: Atomic diffusion in the presence of weak mass loss can explain the observed abundance anomalies of pre-main-sequence stars, as well as the presence of binary systems with metal rich primaries and chemically normal secondaries such as V380 Ori and HD 72106. This is in contrast to turbulence models which do not allow for abundance anomalies to develop on the pre-main-sequence. The age at which anomalies can appear depends on stellar mass. Conclusions: For A and F stars, the effects of atomic diffusion can modify both the internal and surface abundances before the onset of the main-sequence. The appearance of important surface abundance anomalies on the pre-main-sequence does not require mass loss, though the mass loss rate affects their amplitude. Observational tests are suggested to decipher the effects of mass loss from those of turbulent mixing. If abundance anomalies are confirmed in pre-main-sequence stars they would severely limit the role of turbulence in these stars.