A simple accretion/diffusion model for Lambda Bootis stars

It has recently been suggested that the peculiar abundance patterns observed in the Lambda Bootis stars could be understood in terms of accretion of gas previously depleted in metals by means of grain formation in the interstellar medium. A simple analytical model is presented, describing the evolution of elemental abundances in these stars, under the combined influence of accretion and chemical separation. The only arbitrary parameter involved is the accretion rate. A rate of order 10 to the {minus}13th solar mass/yr is found to naturally reproduce many peculiar characteristics of Lambda Bootis stars, in particular their restriction to the spectral type range A0{minus}F0. This lends quantitative support to the accretion hypothesis as a key component toward an understanding of the Lambda Bootis phenomenon. 18 refs.