Non-turbulent pulsation: Metallicism in pulsating stars

In the late 1970s after I showed that evolved Am stars can also pulsate near the red edge of the instability strip, Cox, King & Hodson calculated that the shift in mass fraction of the He II ionization zone should be enough to start pulsation in an Am star which has already undergone diffusion. No one has had much to say theoretically about this problem since then, although I have continued to find more extreme objects which both pulsate and have strong abundance anomalies. The latest is HD 40765 which shows that pulsation with surface radial velocity amplitudes of 10 km s(-1) can coexist with evolved-Am (rho Pup) abundance anomalies. We know that the abundance peculiarities in the Am and Ap star are confined to a thin surface layer. The co-existence of pulsation and strong abundance anomalies demands that the pulsation be nonturbulent, i.e. completely laminar. For HD 40765, thismeans that 10 km s(-1) pulsation generates no turbulence at the level of a fraction of a cm s(-1).