Particle transport and the Lambda Bootis phenomenon. II - an accretion/diffusion model

The surface and internal abundance evolution in a star accreting metal-depleting material from the interstellar medium are described using a set of numerical calculations. The transport model takes into account contributions from chemical separation mechanisms, rotationally induced meridional circulation, and accretion. Computations demonstrate that the maintenance of the abundance signature of the accreted materials is possible only for accretion rates larger than few 10 exp -14 solar masses/yr. Upon termination of the accretion episode, chemical separation destroys any surface abundance pattern set up earlier by accretion during the period of 10 exp 6 yr.