Deep Mixing at Variable Speed in Stars: Is Thermohaline Diffusion Sufficient?

We study the circulation of matter in red giants above the H-burning shell, which is known to yield the appearance at the stellar surface of p-capture isotopes like 7Li, 13C, 17O and the unstable 26Al. These isotopes were observed (either in presolar grains of circumstellar origin or in the photospheres of evolved stars) to display abundance ratios to other nuclei that cannot be accounted for by canonical stellar models. Slow mixing below the convective envelope is the usual explanation invoked for their abundance. Diffusion generated by an inversion in the molecular weight mu is today the most commonly assumed driving mechanism for it. We argue that slow transport reaching moderate temperatures (T < 4×107 K), like the one achievable by diffusive processes induced by a mu inversion can account for some, but not all the observational constraints. In particular the production of Li after the first dredge up and of 26Al in the final evolutionary stages both call for substantially different mechanisms. We show how the buoyancy of magnetic instabilities from toroidal flux tubes can offer a paradigm where not only the production and destruction of Li, but also the appearance of all the other known abundance peculiarities, including the presence of 26Al, can be accounted for. Even in the case that thermohaline mixing were recognized as a necessary byproduct of shell-H burning, it would be nevertheless clear that evolved stars must also host other transport processes, whose effects would therefore combine in the production of the isotopes of light and intermediate elements.