4. Diffusion and transport processes
For the Sun, it is now well established that microscopic diffusion by gravitational settling has a measurable impact on models thus on their pulsation spectrum. As time goes on, all chemical species sink in the gravitational well, except the lightest one, . At the surface, the amount of hydrogen is enhanced while helium and heavy elements are depleted. This implies a decrease of the surface metallicity with respect to time. Since the time-scale of abundance variations in the convection zone is roughly proportional to the square root of the mass of the convection zone (Michaud 1977), the microscopic diffusion time-scale decreases with increasing stellar mass implying that diffusion is more efficient in massive stars. As a consequence, a smaller surface metallicity is expected for Cen A than for Cen B. This is an additional constraint for the models.
As lithium burns at temperatures close to 3 MK, the observed surface abundance is often used as a probe for theories of transport processes at work beneath the external convective zone. The microscopic diffusion alone is not efficient enough to explain the lithium depletion observed in the Sun and stars; thus, there is a need for an unknown physical process, e.g. a turbulent mixing, acting in the radiative zone just beneath the outer convection zone - e.g. Schatzman (1996), Montàlban & Schatzman (2000) for a review. According to the present state of the art, either the shear resulting from the different rotational status between the convection zone and the radiative interior (e.g. Zahn 1992), or internal waves (e.g. Montàlban & Schatzman 2000), are believed to be responsible for that turbulence. For stars more massive than the Sun, turbulent mixing is required, in some (hypothetical) mixed stellar mass fraction, to connect and to extend somehow the mixing of the external convection zones. This avoids the complete segregation of helium and heavy elements from the surface and can explain the AmFm phenomenon (e.g. Richer et al. 2000). Despite numerous attempts, there is today no fully satisfactory prescription for turbulent diffusion able to account for the observed solar lithium depletion; for other stars the situation is even worse ! Further information on the mixing processes at work below the outer convection zones can be provided by simultaneous observations of lithium and beryllium, this latter being depleted deeper in the stars at temperatures slightly larger than for lithium.
© European Southern Observatory (ESO) 2000
Online publication: December 11, 2000