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On the problem of cohabitation between AM and delta-Scuti or 'normal' A stars - A unified theory
A previous interpretation (Baglin, 1972) of the occurrence of deltaScuti variables and Am stars in the same region of the H-R diagram foundan order of magnitude of the maximum rotational velocity that allows thestars to appear as Am. These stars are observed as nonpulsating stars asa consequence of the downward diffusion of helium. For higher rotationalvelocity, the turbulence in the external layers, due to the shearinstability of the meridional circulation, ensures a homogeneouschemical composition. It does not allow the diffusion processes to actefficiently and preserves the ability of the He ionization zone toenergize the pulsations, with the star appearing as a delta Scutivariable. This paper takes an alternate approach, assuming the rotationinduced by the meridional circulation to be time dependent and derivingReynolds and Richardson numbers for this differential rotation. It isshown, among other things, that the differential rotation maintains itsstability even if its Reynolds number is supercritical, as long as thedensity stratification is able to stabilize any perturbation accordingto the Richardson criterion.
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