PARTICLE-TRANSPORT AND THE LAMBDA BOOTIS PHENOMENON .1. THE DIFFUSION MASS-LOSS MODEL REVISITED

Recent observational work has cast doubt on the validity of the diffusion-based model originally developed to account for the peculiar characteristics of the lambda Bootis stars. Among some of the difficulties facing the model, the potential effects of rotation stand most prominently. The original diffusion model neglects all forms of rotationally induced mixing, be it in the form of turbulence or large-scale meridional circulation. It was already realized then that this may well be a critical omission, as meridional circulation proved to play an important role in the evolution of chemical abundances in other types of chemically peculiar stars. 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.