Magnetic mode depression and frequency shifts in quadrupole modes of red giants
Abstract
When describing the effects of core-magnetic fields on the oscillation modes in red giants, we can distinguish two regimes: weak or strong magnetic field. The threshold field strength is the so-called critical field strength. For strong fields, we expect the energy of the mode to be fully dissipated in the g-mode cavity. For weak fields, we expect the mixed-mode frequencies to be shifted to higher frequencies. As the critical field strength decreases with spherical degree, a subset of red giants should have magnetic frequency shifts in their dipole modes, while the g-mode contribution in their quadrupole modes is fully dissipated. In this regard, finding stars in this specific magnetic field regime is valuable, as these stars are important testing grounds for current understanding of core-magnetism. To this end, we analysed the power spectra of eleven stars with confirmed magnetic shifts and no clear signs of rotation (allowing for a precise determination of the magnetic shift). From their quadrupole-mode properties, we deduce that eight stars exhibit signs of full dissipation of the g-mode character, while three others show only partial dissipation. For the eight stars, their magnetic field estimates confirm that the quadrupole-mode dissipation can be linked to the presence of a strong core-magnetic field. For two of the three stars with partial dissipation, we can also explain their mode properties through the presence of a core-magnetic field. Our results directly link the mode-energy dissipation in non-radial modes to the presence of a core-magnetic field. Moreover, we confirm that the combination of dipole and quadrupole-mode properties can help us confirm the presence of core-magnetic fields and understand them.