Inhibition of thermohaline mixing by a magnetic field in Ap star descendants:: implications for the Galactic evolution of 3He

Aims. To reconcile the measurements of (3)He/H in Galactic HII regions with high values of (3)He in a couple of planetary nebulae, we propose that thermohaline mixing is inhibited by a fossil magnetic field in red giant stars that are descendants of Ap stars. Methods. We examine the effect of a magnetic field on the salt-finger instability, using a local analysis. Results. We obtain a threshold for the magnetic field of 10(4)-10(5) Gauss, above which it inhibits thermohaline mixing in red giant stars located at or above the bump. Fields of that order are expected in the descendants of the Ap stars, taking into account the contraction of their core. Conclusions. We conclude that in a large fraction of the descendants of Ap stars thermohaline mixing does not occur. As a consequence these objects must produce (3)He as predicted by the standard theory of stellar evolution and as observed in the planetary nebulae NGC 3242 and J320. The relative number of such stars with respect to non-magnetic objects that undergo thermohaline mixing is consistent with the statistical constraint coming from observations of the carbon isotopic ratio in red giant stars. It also satisfies the Galactic requirements for the evolution of the (3)He abundance.