Abstract

One of the “legacy” results of the Kepler mission is the interestingly low rotation rate of the core of subgiant (SG) and red giant (RG) stars, which is about 10 times lower than predicted with the current theory for the transport of angular momentum by purely hydrodynamical mechanisms. This discrepancy points out an order of magnitude issue concerning the understanding of the evolution of the stellar angular momentum in evolved Solar-like stars, a very ubiquitous problem shared by stars of all types and ages. The recent discovery of very low-amplitude dipolar oscillations in a significant fraction of the observed SGs and RGs also points out our misunderstanding of physical processes inside the radiative interiors of evolved Solar-like stars. We thus seek for a missing process taking place inside the core of evolved Solar-like stars, efficient to extract angular momentum from the core to the surface and to perturbate stellar oscillations.

Internal magnetic fields are one amongst the most serious candidates that are currently studied to solve both problems. Stars more massive than ~1.1 Ms are known to develop a convective core during the main-sequence: the dynamo process due to this convection could be the origin of a strong magnetic field, trapped inside the core of the star for the rest of its evolution. Such magnetic fields should impact mixed modes inside the core of RG stars, and their signature should be visible in asteroseismic data. To unravel which constraints can be obtained from these observations, we theoretically investigate the effects of a plausible mixed axisymmetric magnetic field with various amplitudes on the mixed-mode frequencies of red giants. Applying a perturbative method, we estimate the magnetic splitting of the frequencies of simulated mixed dipolar modes that depends on the magnetic field strength and its configuration. A complete asymptotic analysis is derived, showing the potential of asteroseismology to probe the magnetism at each depth as this is done for stellar rotation. The effects of the mass and the metallicity of the stars are also explored. Finally, we infer an upper limit for the strength of the field and the associated lower limit for the timescale of its action to redistribute angular momentum in stellar interiors.