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

TL;DR

The study investigates whether fossil magnetic fields can explain the magnetic fields observed on old white dwarfs by linking recent asteroseismic detections of internal red-giant fields to surface WD magnetism. It models three evolution scenarios for the field and evolves the field through stellar evolution with diffusion and flux conservation using MESA structure, calibrating against RG asteroseismic measurements. The key result is that a stable field filling the radiative interior during the RGB (Scenario C) can produce surface WD fields consistent with observations, whereas fields from MS convective cores (A/B) are likely buried and fail to reproduce breakout timescales. This supports the fossil-field scenario as a viable explanation for WD magnetism and emphasizes the need for magnetized radiative interiors during RGB to connect RG cores to WD surfaces.

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.

Figures (6)

• Figure 1: Considered magnetic field configurations at different evolutionary stages of a typical $1.5\text{M}_\odot$ star. The central field strengths are set such that asteroseismic detections of all three fields would measure 100kG during the RGB (see App. \ref{['app:kernel_avg']}). Left panel: Scenarios A (darkblue line) and B (lightblue line) are created by the convective core during the MS and start evolving as a large-scale stable field at the end of the MS. Middle panel: Magnetic fields during the RGB when the typical oscillation frequency of the star reaches $150\mu\text{Hz}$. The two fields for scenarios A and B are now buried below the Hydrogen-burning shell (dashed line). Scenario C (red line) starts its evolution here and fills the entire radiative interior. Right panel: Magnetic field configurations for all three scenarios at the start of the WD cooling sequence. The blue shaded region corresponds to the radial extent of the WD mass at different evolutionary stages.
• Figure 2: Radial surface field strength as a function of cooling age of the WD for scenarios B (blue) and C (red). The shaded regions show the evolution for varying field strengths on the RGB between $10$ (bottom) and $200$ (top) kG, as the range detected in Hatt2024. The dashed line shows the emergence of the strongest detected RGB field of $600$kG Deheuvels2023 and places a lower limit on the FFS emergence timescale. The white circles show detected magnetic WDs from the sample of Bagnulo2022 in the mass range $[0.5,0.64]\text{M}_\odot$.
• Figure 3: Comparison between $b_r(r)$ and $b_\theta(r)$ (Eqs. \ref{['eq:br_def']}-\ref{['eq:bt_def']}) from Broderick2007 (dotted lines) and the polynomial fit that enforces the correct boundary conditions (full lines). The fit is created such that both formalisms agree at half the strength of $b_r(r)$ (black dashed line).
• Figure 4: Evolution of Scenario C during the RGB around the hydrogen-burning shell (solid black line). The magnetic mass $\mathcal{M}_C\sim0.26\text{M}_\odot$ is shown as a solid red line. Left: Contour map of the magnetic flux in log-scale as it evolves according to Eq. \ref{['eq:final']}. Middle: Contour map of $r^{-2}$ in log-scale. Right: Contour map of $b_r$ in log-scale as a result of multiplying the two panels above. The dashed black line shows the location of the maximum of $b_r$.
• Figure 5: A 2D slice of the magnetic field of Scenario C at the start of the WD cooling sequence. Left: The local magnetic field strength $\sqrt{\Vec{B}\cdot\Vec{B}}$ throughout the full WD. The yellow region shows where most of the magnetic field strength is located. Right: The corresponding field lines to the same magnetic field shown on the left.