PARTICLE-TRANSPORT AND THE LAMBDA BOOTIS PHENOMENON .2. AN ACCRETION DIFFUSION-MODEL

We present a large set of numerical calculations describing the surface and internal abundance evolution in a star accreting metal-depleted material from the interstellar medium. The transport model includes contributions from chemical separation mechanisms, rotationally induced meridional circulation, and accretion. Calculations are performed in a T(eff) = 8000 K main-sequence model, for a few representative chemical species. While the main purpose of the paper is to put the accretion/diffusion model for lambda Bootis stars on a firmer quantitative footing, many of the conclusions drawn are directly transferable to accretion on '' normal '' main-sequence A stars. Our computations demonstrate the following: (1) the maintenance of the abundance signature of the accreted material is only possible for accretion rates larger than few 10(-14) M. yr-1, the exact value being function of the chemical element under consideration. (2) Upon terMination of the accretion episode, chemical separation destroys any surface abundance pattern set up earlier by accretion, and does so in as little as 10(6) yr. (3) For some elements, however, a certain level of memory of the accretion phase persists to late epochs, in that the asymptotic overabundance levels are smaller than if accretion had not occurred. (4) For stars with equatorial rotational velocities of 125 km s-1 or less, and accreting at a rate of 10(-13) M. yr-1, rotationally induced meridional circulation has no significant influence on the evolution of surface abundances, during the accretion episode itself. But upon termination of accretion, it further contributes in erasing the effects of the accretion episode. While the present computations lend further support to the accretion/diffusion model proposed recently for A Bootis stars, they also establish a number of constraints on the model. In particular, our results show that if the peculiar abundance pattern characterizing the A Bootis phenomenon is due to the differential accretion of metal-depleted material, then we are not seeing the residual signature of a long-gone accretion episode, such as during star formation itself or early on the pre-main-sequence. Instead, accretion must be ongoing or have ceased within the last 10(6) yr or so. This suggests that nearly all A Bootis stars should show observational evidence for the presence of circumstellar material.