On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: II- Application to light elements in population I main-sequence stars

In the two previous papers of this series, we have discussed the importance of t he $\mu$-gradients due to helium settling on rotation-induced mixing, first in a n approximate analytical way, second in a 2D numerical simulation. We have found that, for slowly rotating low mass stars, a process of ``creeping paralysis" in which the circulation and the diffusion are nearly frozen may take place below the convective zone. Here we apply this theory to the case of lithium and beryll ium in galactic clusters and specially the Hyades. We take into account the rota tional braking with rotation velocities adjusted to the present observations. We find that two different cells of meridional circulation appear on the hot side of the "lithium dip" and that the "creeping paralysis" process occurs, not dir ectly below the convective zone, but deeper inside the radiative zone, at the to p of the second cell. As a consequence, the two cells are disconnected, which ma y be the basic reason for the lithium increase with effective temperature on thi s side of the dip. On the cool side, there is just one cell of circulation and t he paralysis has not yet set down at the age of the Hyades; the same modelisatio n accounts nicely for the beryllium observations as well as for the lithium ones .Comment: 13 printed pages, 10 figures. ApJ, in press (April 20, 2003

On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: I- Analytical Approach

In the presence of rotation-induced mixing, element diffusion still occurs in stellar radiative zones, although at a slower rate than in the case of a complete stability of the stellar gas. As a consequence, helium settling leads to vertical $\displaystyle \mu$-gradients which, due to the meridional circulation, turn into horizontal fluctuations. Up to now, the feed-back effect of this process on the rotation-induced mixing was currently neglected in the computations of abundance variations in stellar surfaces, or artificially reduced. Here we analyse its consequences and derive an approximate analytical solution in a quasi-stationary case. We also discuss the relative importance of the various physical effects which influence the meridional circulation velocity. In a second paper (Th\'eado and Vauclair 2002a), we will present a complete 2D numerical simulation of this process while a third paper (Th\'eado and Vauclair 2002b) will be devoted to special applications to Pop I stars.Comment: 7 printed pages, 1 figure. ApJ, in press (April 20, 2003

Instabilités hydrodynamiques dans les étoiles en rotation en relation avec l'hélio- et l'astéro-sismologie et les contraintes imposées par l'observation des éléments légers

TOULOUSE3-BU Sciences (315552104) / SudocMEUDON-Observatoire (920482302) / SudocSudocFranceF

Influence of Thermohaline Convection on Diffusion-Induced Iron Accumulation in a Stars

International audienceAtomic diffusion may lead to heavy-element accumulation inside stars in certain specific layers. Iron accumulation in the Z-bump opacity region has been invoked by several authors to quantitatively account for abundance anomalies observed in some stars, or to account for stellar oscillations through the induced kappa-mechanism. These authors, however, never took into account the fact that such an accumulation creates an inverse mu-gradient, unstable for thermohaline convection. Here, we present results for A-F stars, where abundance variations are computed with and without this process. We show that iron accumulation is still present when thermohaline convection is taken into account, but much reduced compared to when this physical process is neglected. The consequences of thermohaline convection for A-type stars as well as for other types of stars are presented