Gyroscopic pumping of large-scale flows in stellar interiors, and application to Lithium Dip stars

摘要

The maintenance of large-scale differential rotation in stellar convectiveregions by rotationally influenced convective stresses also drives large-scalemeridional flows by angular--momentum conservation. This process is an exampleof ``gyroscopic pumping'', and has recently been studied in detail in the solarcontext. An important question concerns the extent to which thesegyroscopically pumped meridional flows penetrate into nearby stably stratified(radiative) regions, since they could potentially be an important source ofnon-local mixing. Here we present an extensive study of the gyroscopic pumpingmechanism, using a combination of analytical calculations and numericalsimulations both in Cartesian geometry and in spherical geometry. The variousmethods, when compared with one another, provide physical insight into theprocess itself, as well as increasingly sophisticated means of estimating thegyroscopic pumping rate. As an example of application, we investigate theeffects of this large-scale mixing process on the surface abundances of thelight elements Li and Be for stars in the mass range 1.3-1.5 solar masses(so-called ``Li-dip stars''). We find that gyroscopic pumping is a veryefficient mechanism for circulating material between the surface and the deepinterior, so much in fact that it over-estimates Li and Be depletion by ordersof magnitude for stars on the hot side of the dip.However, when the diffusionof chemical species back into the surface convection zone is taken intoaccount, a good fit with observed surface abundances of Li and Be as a functionof stellar mass in the Hyades cluster can be found for reasonable choices ofmodel parameters.