On the problem of cohabitation between AM and delta-Scuti or 'normal' A stars - A unified theory

Details On the problem of cohabitation between AM and delta-Scuti or 'normal' A stars - A unified theory On the problem of cohabitation between AM and delta-Scuti or 'normal' A stars - A unified theory

Aug 1976

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976a%26a....50..435v&link_type=abstract

Astronomy and Astrophysics, vol. 50, no. 3, Aug. 1976, p. 435-444.

Astronomy and Astrophysics

Astrophysics

51

A Stars, Abundance, Stellar Rotation, Stellar Structure, Variable Stars, Helium, Particle Diffusion, Radial Velocity, Stellar Evolution, Turbulent Diffusion

Scientific paper

A previous interpretation (Baglin, 1972) of the occurrence of delta Scuti variables and Am stars in the same region of the H-R diagram found an order of magnitude of the maximum rotational velocity that allows the stars to appear as Am. These stars are observed as nonpulsating stars as a consequence of the downward diffusion of helium. For higher rotational velocity, the turbulence in the external layers, due to the shear instability of the meridional circulation, ensures a homogeneous chemical composition. It does not allow the diffusion processes to act efficiently and preserves the ability of the He ionization zone to energize the pulsations, with the star appearing as a delta Scuti variable. This paper takes an alternate approach, assuming the rotation induced by the meridional circulation to be time dependent and deriving Reynolds and Richardson numbers for this differential rotation. It is shown, among other things, that the differential rotation maintains its stability even if its Reynolds number is supercritical, as long as the density stratification is able to stabilize any perturbation according to the Richardson criterion.

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