Element settling in the solar interior

Element settling inside the Sun now becomes detectable from the comparison of the observed oscillation modes with the results of the theoretical model s. This settling is due, not only to gravitation, but also to thermal diffu sion and radiative acceleration (although this last effect is small compare d to the two others). It leads to abundance variations of helium and heavy elements of congruent to 10% below the convective zone. Although not observ able from spectroscopy, such variations lead to non-negligible modification s of the solar internal structure and evolution. Helioseismology is a power ful tool to detect such effects, and its positive results represent a great success for the theory of stellar evolution. Meanwhile, evidences are obta ined that the element settling is slightly smoothed down, probably due to m ild macroscopic motions below the convective zone. Additional observations of the abundances of both Li-7 and He-3 lead to specific constraints on the se particular motions.