Magneto-Archeology of White Dwarfs. Revisiting the fossil field scenario with observational constraints during the red giant branch

TL;DR

This study investigates whether fossil magnetic fields in red giants can explain the strong magnetic fields observed in old white dwarfs, using observational constraints and magnetic evolution modeling.

Contribution

It revisits the fossil field hypothesis by modeling magnetic flux evolution and diffusion from red giants to white dwarfs, constrained by asteroseismic observations.

Findings

Red giant magnetic fields are compatible with white dwarf magnetic strengths and emergence times.

Fields from a convective-core dynamo are too deep to explain white dwarf magnetism.

A magnetized radiative zone during the red giant phase is crucial for the fossil field scenario.

Abstract

The detection of strong, large-scale magnetic fields at the surface of only the oldest population of white dwarfs might point towards 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 revisit the fossil field framework by using the asteroseismic detections to constrain the strength of such magnetic fields as they evolve to the white dwarf stage. We assume that the magnetic field was either created during the main sequence core convection or that it fills the radiative interior as the star evolves on the red giant branch. From these, we evolve the magnetic flux, allowing for magnetic diffusion along the evolution of a 1.5Msun modelled 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 during the red giant branch would be buried too deep in the star and not match the breakout timescales and the field strengths of magnetic white dwarfs. A broadly magnetized internal radiative zone during the red giant branch is therefore key for the fossil field theory to connect magnetic fields observed along the late evolution of stars.