LIGHT-INDUCED DRIFT AND ABUNDANCE ANOMALIES

Light-induced drift (LID) has been suggested by Atutov and Shalagin to be more efficient at causing abundances anomalies than radiation driven diffusion. A formalism is developed to simplify the accurate evaluation of LID in stars. The LID velocity is written in such a way that the effect of LID can easily be compared to the effect of the radiative acceleration of the same transition of an element. We also compare approximations that facilitate the evaluation of this phenomenon. Our calculations show that LID accelerates the separation of He-3 from He-4 in the atmosphere of a main-sequence star with T(eff) = 18,000 K. While He-4 is present, LID increases the upward drift of He-3 thus causing the relative overabundance of He-3 to appear earlier in the evolution. As He-4 settles gravitationally, the anisotropy disappears in the lines of He-3 and it is supported only by radiation pressure so that the same final abundance is maintained in the atmosphere as when LID is neglected. The effect was also calculated for Li III at the base of the convection zone of a star with T(eff) = 6700 K. At this depth, LID is about two orders of magnitude smaller than radiatively driven diffusion, thus playing a negligible role in the transport of this element. LID appears to be negligible for oxygen in A type stars, but more precise calculations are needed to confirm this result. It is negligible for Hg in B stars.