Hydrodynamic mixing processes are modelled very crudely in stellar-evolution codes, although they
have a significant impact on stellar evolution and nucleosynthesis. There is ample observational
evidence that some form of mixing extends beyond the formal boundaries of stellar convection zones.
This "overshoot" can be provided by several physical mechanisms, depending on the type of
convection zone in question. I will focus on two of them, which have been paid little attention to
so
far: the "convective settling" process could explain the lithium depletion observed in low mass
stars, while the "differential heating" process could provide some mixing at the boundaries of
stellar convective cores and shells.
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