Short Period Variable Stars IX. Rotation and Mixing in the Outer Layers of A Stars. Turbu- lent Mixing Due to the Meridional Circulation Velocity Field

It is shown that mixing (or non mixing) due to the meridional circulation velocity field associated with fast (or slow) rotation can explain most of the key observational properties of A stars (Breger, 1970). At present, it seems that the data are all consistent whith the following scheme: slow rotators have abundance anomalies and are vibrationally stable, fast rotators are variable. When the equatorial velocity is smaller than 50 km/s abundances difference can develop and inhibit vibrational instability. When it is somewhat larger than 50 km/s turbulent mixing forces the star to remain homogeneous so that the helium content of the envelope is sufficient to drive the instability associated with the so-called " mechanism". Key words: stellar evolution - rotation - variability