Extra-mixing in AGB Stars from magnetic buoyancy

We study the circulation of matter in AGB stars above the H-burning shell, which is known to yield the appearance of p-rich isotopes like 13C, 17O and the unstable 26 Al at the stellar surface. These nuclei were observed in presolar grains of AGB origin and in some cases (e.g. C isotopes) in spectroscopic observations of evolved stars. For the physical mechanism driving the mixing we consider magnetic flux tube buoyancy. Magnetic tubes are formed below the convective envelope and one can express the parameters of the required mixing phenomena in terms of the magnetic field intensityj ! Bj. We show that the required values ofj ! Bj can span a considerable range. If the mixing arrives only at the innermost layers of the convective envelope, fields smaller than the solar ones are su cient, according to our previous analysis. If instead magnetic fields have to appear at the surface, as is suggested by recent observations, than field intensities in the Megagauss range are necessary.