Particle transport and the Lambda Bootis phenomenon. I - The diffusion/mass-loss model revisited

In this paper, results of two-dimensional linear time-dependent particle transport calculations in a Lambda Bootis star are presented for two representative elements, titanium and calcium. These demonstrate unambiguously that the inclusion of meridional circulation in the original diffusion/mass-loss model of Lambda Bootis stars has profound consequences on the abundance evolution. More specifically, circulation prevents the appearance, at any epoch of main-sequence evolution, of the underabundance patterns characteristic of Lambda Bootis stars. This indicates that the diffusion/mass-loss model for these objects must be either abandoned or significantly modified. Caveats and possible alternatives are discussed, and a few observational tests are suggested.