Evolution of a Long-Lived Deep-Seated Main-Sequence Magnetic Field During White Dwarf Cooling

TL;DR

This study models the evolution of deep-seated magnetic fields in white dwarfs originating from main-sequence core dynamos, explaining observed magnetic field features and their dependence on mass and age.

Contribution

It introduces a comprehensive framework combining stellar evolution, WD cooling, and magnetic diffusion to explain magnetic field development in white dwarfs from main-sequence dynamos.

Findings

Stronger magnetic fields emerge earlier in more massive WDs.

Surface magnetic field evolution is slower in less massive WDs due to deeper magnetic boundary.

Predicted magnetic field strengths align with MHD dynamo simulations and equipartition scaling.

Abstract

We study the evolution of white dwarf (WD) magnetic fields that originate from core-convective dynamos during the main-sequence. Using stellar evolution and WD cooling models combined with magnetic field diffusion calculations, we demonstrate that a surviving field from the main-sequence can account for various features observed in magnetic WDs. In particular, the earlier emergence of stronger magnetic fields in more massive WDs, compared to older, less massive, and less magnetic ones, can be explained by this framework. This is because the magnetic boundary at the onset of WD cooling lies deeper in less massive WDs, resulting in a slower and weaker evolution of the surface magnetic field due to increasing electrical conductivity over time. We further show that many of the magnetic field strengths observed across different WD samples can be reproduced if the deep-seated field generated during the main sequence is comparable to predictions from magnetohydrodynamic simulations of core-convective dynamos, or if equipartition provides a valid scaling for the main-sequence dynamo. Additionally, our predictions for surface magnetic fields vary by a factor of 2 to 4 when higher-order modes of poloidal magnetic field expansion and turbulent diffusion driven by crystallization-induced convection are included. These effects should therefore be considered when investigating the origin of magnetic fields in individual WDs.