TURBULENCE AND THE LI ABUNDANCE IN MAIN-SEQUENCE AND GIANT STARS

The Li abundance in cluster giants serves as a probe of turbulent transport in main-sequence stellar envelopes. Three parameterizations proposed for the turbulent diffusion coefficient are used to compute the level of lithium depletion in first-ascent giants. A full time-dependent solution of the transport equation is performed, explicitly including nuclear reaction terms, and taking into account evolutionary effects. It is found that, with an appropriately chosen parameter, turbulence can lead to a level of particle transport very similar to that associated with meridional circulation, and thus to the extreme Li underabundances recently observed in giants of M67 and NGC 752. However, the Li abundances in NGC 7789 giants are not compatible with turbulent transport. We also obtain numerical solutions with the turbulent diffusion coefficient used recently by Vauclair to model the Hyades Li abundance gap; this model is characterized by the presence of a turbulent-free zone, which is assumed to shield the Li located above it from nuclear burning occurring below. The inclusion of ordinary microscopic diffusion in the transport equation is sufficient to lead to a significant particle flux across the turbulent-free zone; this then leads to large Li underabundances on the hot side of the Li gap, contrary to observations. Finally, it is pointed out that two of these models for turbulent transport lead, in more massive stars, to levels of turbulence under the He superficial convection zone which are incompatible with the requirement that He settling be efficient in FmAm stars.