Magneto-archeology of white dwarfs. Revisiting the fossil field scenario with observational constraints during the red giant branch

The detection of strong, large-scale magnetic fields at the surfaces of the oldest white dwarfs might point toward a hidden internal magnetic field slowly rising to the surface. In addition, strong magnetic fields have recently been measured through asteroseismology in the radiative interiors of red giant stars, the progenitors of white dwarfs. To investigate the potential connection between these observations, we revisited the fossil field framework using asteroseismic detections to constrain the strength of such magnetic fields as red giants evolve into the white dwarf stage. We assumed that the magnetic field was either created during the core convection on the main sequence or that it fills the radiative interior as the star evolves on the red giant branch. From these initial conditions, we evolved the magnetic flux, allowing for magnetic diffusion along the evolution of a modeled 1.5Msun star. We find that measured field strengths in red giants attributed to the hydrogen-burning shell are compatible with the field amplitudes and emergence timescales of magnetized white dwarfs. On the contrary, magnetic fields generated solely from a convective-core dynamo on the main sequence and detectable on the red giant branch would be buried too deep in the star and would not match the breakout timescales or the field strengths of magnetic white dwarfs. Therefore, for us to connect magnetic fields observed along the late evolution of stars via a fossil field we would need to find a broadly magnetized internal radiative zone on the red giant branch.