Meridional circulation and diffusion in A and early F stars

Time-dependent two-dimensional calculations of diffusion in the presence of meridional circulation are presented for stellar models pertaining to FmAm stars. It is shown that, once the helium superficial convection zone (HSCZ) has disappeared, the meridional circulation has little influence on chemical separation. In stars rotating too rapidly to become FmAm stars, chemical separation remains possible under the HSCZ. Meridional circulation does not completely wipe out chemical separation at these velocities, and cannot by itself lead to the abundance patterns characteristic of Lambda Booti stars. Upper limits to turbulence are set. In the presence of meridional circulation, helium settling in stars rotating at the observed cutoff for FmAm stars remains possible for values of vertical turbulent diffusion coefficients of order 1000 sq cm/s under the helium convection zone. This sets extremely tight constraints on turbulence in stars with equatorial rotational velocities of 100 km/s or less.