The Effect of Diffusion on Pulsations of Stars on the Upper Main   Sequence. delta Scuti and Metallic A Stars
  Recent dramatic improvements in the modeling of abundance evolution due to diffusion in A stars have been achieved with the help of monochromatic opacity tables from the OPAL group. An important result in the context of stellar pulsations is the substantial helium abundance shown to be left over in the driving region of delta Scuti-type pulsations in chemically peculiar Am stars. An accurate opacity profile in the entire stellar envelope including the full effect of heavy elements is also now available for the first time.   Pulsations are shown to be excluded for young Am stars but occur naturally when these stars evolve off the ZAMS. The predicted variable metallic A stars all lie towards the red edge of the instability strip, in qualitative agreement with the observed variable delta Delphini and mild Am stars.   Results show little direct excitation from iron-peak elements in A-type stars. The main abundance effect is due to the settling of helium, along with a marginal effect due to the enhancement of hydrogen. 
S. Turcotte, J. Richer, G. Michaud, and J. Christensen-Dalsgaard