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

TL;DR

This paper proposes that magnetic fields in Ap star descendants inhibit thermohaline mixing in red giants, explaining observed 3He abundances and aligning stellar evolution models with galactic chemical evolution data.

Contribution

It introduces a magnetic field threshold that prevents thermohaline mixing in certain red giants, providing a new explanation for 3He observations and stellar evolution processes.

Findings

Magnetic fields of 10^4 - 10^5 Gauss inhibit thermohaline mixing.

Most Ap star descendants do not undergo thermohaline mixing.

Results align with observed 3He abundances and galactic chemical evolution constraints.

Abstract

To reconcile the measurements of 3He/H in Galactic HII regions with high values of 3He 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. We examine the effect of a magnetic field on the salt-finger instability, using a local analysis. 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. 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 3He as predicted by the standard theory of stellar evolution and as observed in the planetary nebulae NGC3242 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 3He abundance.