The element settling which occurs inside stars, due to the combined effect of gravity and thermal gradient (both downwards), radiative transfer (upwards) and concentration gradients, leads to abundance variations which cannot be neglected in computations of stellar structure. This process is now generally introduced as a “standard process” in stellar evolution codes. The new difficulty is to explain why, in some cases, element settling does not proceed at all as expected. Macroscopic motions, like rotation-induced mixing, may increase the settling time scales, but then it introduces in radiative regions extra mixing with consequences which are not always observed as predicted. We have recently developed a new approach for treating rotation-induced mixing in which we include the feedback effect of the settling-induced μ-gradients (Vauclair 1999, Théado & Vauclair 2001, 2002). This effect, which was not included in previous computations, leads to first order terms in the meridional circulation velocity. It results in a mixing process, just below the convective zone, quite different from that induced by normal circulation. For the first time, we have evidence of a mixing region which is precisely confined and directly modulated by the settling itself. This will have interesting consequences for the computations of abundance variations in stars.

Introduction

Although element settling inside stars was already recognized as a fundamental process at the very beginning of the computation of stellar structure and evolution (Eddington 1926), it has long been forgotten by stellar astrophysicists.