3. Metallicism and pulsations: theory
Elements migrate with respect to each other because of differential forces mostly due to inward gravity and outward radiative pressure. This segregation of different atomic species is what is termed here diffusion. Diffusion is a rather fragile process because typical diffusion velocities are of the order of fractions of cm per second. Large-scale motion of matter or turbulence quickly overwhelm diffusion and homogenize the chemical composition outside of nuclear burning regions. Diffusion is efficient only in stars where the competing processes are weak. This explains why CP stars are mainly of spectral types ranging from early F to late B-type stars. In these spectral types the surface convection zone is thin enough and the mass loss rates small enough (a mass loss of around is large enough to remove all anomalies) to permit the development of significant abundance anomalies. As a result, CP stars are typically relatively unevolved, slowly rotating stars. There are some CP stars, Ap or Bootis stars, for example, in which other factors come into play.
In the standard picture for FmAm, HgMn and Ap stars, the observed abundance anomalies of heavy elements develop as a consequence of the settling of helium. As it disappears from the superficial convection zone it can no longer provide the opacity to sustain convection in the HeII zone and the result is a much thinner convection zone due to the ionization of HI . At this depth, the radiative forces on the various heavy elements are compatible with the pattern of surface-abundance anomalies. This model agrees qualitatively with observations but requires additional assumptions to obtain a quantitative agreement as the predicted anomalies are generally too large. No calculation based on this model has ever reproduced the abundances of individual Am stars.
The major consequence of this model for stellar stability is that helium is no longer present to excite pulsations typical of the classical instability strip. Many studies related to this problem have been carried out. The most thorough, by Cox et al. (1979), showed that variability is possible with a low helium content but that the width of the instability strip decreases as the helium abundance decreases. Their conclusion was that classical Am stars should not be variable but that metallicism and variability were not mutually exclusive in the red part of the classical instability strip if the surface helium abundance fell marginally below 0.1 but without dropping to a very low value.
When the diffusion of heavy elements is included in a consistent fashion this picture changes drastically. These models are dubbed here The New Montreal Models (NMM).
Magnetic Ap stars and Bootis stars will be ignored at this time. We note in passing that for roAp stars a few models have been proposed based on the so-called mechanism, where the dominant driving comes from the effects of the opacity . As only microscopic diffusion can explain the observed abundances at present, the proposed models include its effects. Amongst the possibilities are: the replenishment of superficial helium by advection from mass loss (Vauclair & Dolez 1990), the replacement of helium by silicon pushed in the driving region by radiative levitation (Matthews 1988), and hydrogen overabundances in the HI ionization zone as a result of helium settling (Dziembowski & Goode 1996). As for Bootis stars, the currently preferred but so far unproven accretion models for these stars assume that the helium abundance would remain normal in the driving region (Turcotte & Charbonneau 1993), accounting for the necessary opacity for the mechanism to work.
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000