Diffusion, meridional circulation, and mass loss in Fm-Am stars

The gravitational settling of helium is calculated in the presence of meriodional circulation. The concentration of Fm-Am stars in binary systems with 2.5<P<100 days is then explained without any arbitrary parameter. Together with the other properties of Fm-Am stars that are explained without arbitrary parameter, this constitutes a strong argument in favor of the diffusion model for those objects. Calculations of helium settling and of the diffusion of heavy elements in the presence of mass loss are also carried out to show that a mass loss rate of 10-15 to 10-14 M/sub sun/ yr-1 is likely in Fm-Am stars. Some weak turbulence may subsist below the hydrogen convection zone. The effects of meridional circulation, mass loss, turbulence, and mixing length illustrate how the appearance of abundance anomalies is determined by the hydrodynamics of the outer stellar envelope. The Fm-Am stars offer an important contact between observations and hydrodynamical calculations. If the mass loss rate is approx.10-13 M/sub sun/ yr-1, stars may spend more of their life as lambda Bootis stars than as Fm-Am stars