Kilogauss magnetic fields from simulations of small-scale dynamo action in the convective envelope of a white dwarf

Abstract

About 20% of white dwarfs are observed to host large-scale magnetic fields, but the origin of white dwarf magnetism remains uncertain. Small-scale turbulent dynamos (SSDs), which efficiently generate magnetic fields in solar and stellar convection simulations, have so far been studied in white dwarfs only through equipartition arguments in one-dimensional models. We therefore investigate whether turbulent convection in white dwarf surface layers can sustain SSD action through local three-dimensional radiation-magnetohydrodynamics simulations of a DA white dwarf with a convective pure-hydrogen atmosphere, including the full convection zone together with the underlying overshoot and stably stratified layers. Starting from a weak seed field of 1 mG, the magnetic energy undergoes exponential amplification before saturating at a magnetic-to-kinetic energy density ratio of about 5.5% at the visible surface, demonstrating that SSD action naturally generates kG-strength magnetic fields in convective white dwarf atmospheres. The resulting magnetic field is characterised by a mixed-polarity small-scale structure, with kG field concentrations contributing about 14% of the total unsigned magnetic flux at the visible surface. Despite a rate of magnetic energy generation amounting to roughly one-seventh of the bolometric flux, no significant modification of the mean stratification is found. Although these fields remain spatially unresolved for observations, they may contribute to spectral line broadening, suggesting that small-scale magnetism in white dwarfs could be more widespread than currently inferred from observations.