White Dwarf Magnetism Links Red Giant Fields, Constraining Fossil Field Strength

Scientists are revisiting the long-standing ‘fossil field’ theory to explain the origin of strong magnetic fields in white dwarf stars, a puzzle that has intrigued astronomers for decades. Lukas Einramhof, Lisa Bugnet, and Leila Magdalena Calcaferro, from the Institute of Science and Technology Austria (ISTA) and Universidad Nacional de La Plata, alongside Lucas Barrault and Srijan Bharati Das, present compelling evidence linking magnetic fields detected within red giant stars to those observed on the surface of their eventual white dwarf remnants. Their research uniquely constrains the evolution of these fields, demonstrating that a broadly magnetized interior during the red giant phase is crucial for explaining the observed strengths and emergence timescales of magnetic white dwarfs , effectively ruling out scenarios where fields originate solely from the star’s core during its main sequence lifetime. This work offers a significant step towards understanding how these stellar magnetic fields are generated and sustained throughout the lives of stars.

This breakthrough reveals that the delayed emergence of magnetism in white dwarfs is not simply a matter of time, but rather a consequence of the magnetic field’s location and evolution within the star. The work opens new avenues for understanding the internal dynamics of stars and the role of magnetic fields in shaping their evolution. By connecting observations of red giants and white dwarfs, the research provides a powerful tool for ‘magneto-archeology’, reconstructing the magnetic history of stars by examining their remnants. Future studies can build upon this foundation to explore the diversity of magnetic field configurations in different stellar populations and to investigate the impact of magnetism on stellar angular momentum and the formation of planetary systems.

Magnetic Flux Evolution in 1.5 Solar Mass

The study employed MESA version 24.08.1 to model a 1.5Msun star, meticulously tracking magnetic flux evolution and accounting for diffusion along the star’s life cycle. Researchers normalised field configurations to align with asteroseismic signatures measured by Hatt et al. (2024), utilising the code magsplitpy and methods detailed in recent literature including Das et al. (2020), Bugnet et al. (2021), and Mathis et al. (2021). This normalisation yielded core-averaged squared radial field strengths of ⟨B²r⟩, with a typical value of 100kG adopted for initial conditions, mirroring observed oscillation modes in red giants. To accurately model field evolution, the team moved beyond simple ohmic diffusion calculations, instead solving the full induction equation: ∂B/∂t = ∇∧(u ∧B) −∇∧(η∇∧B).
This approach, following Takahashi & Langer (2021), accounted for changes in stellar structure due to nuclear burning, particularly the strong density gradients around the hydrogen-burning shell. Using MESA, scientists calculated structure parameters and magnetic diffusivity, employing prescriptions from Potekhin et al. (1999) and Stygar et al. (2002) for degenerate and non-degenerate regions, respectively, with linear interpolation between them. The team deliberately neglected the minor impact of white dwarf core crystallisation on magnetic diffusivity to streamline calculations and focus on primary effects. This configuration, absent in other scenarios, resulted in a peak field strength in a shell rather than the core, a key finding supported by detailed analysis in Appendix E of the work. Even higher red giant field strengths, reaching 600kG as detected by Deheuvels et al. (2023), accounted for a significant portion of remaining magnetic white dwarfs, solidifying the link between late-stage stellar evolution and observed magnetic phenomena.

Red Giant Fields Mirror White Dwarf Magnetism

The research team meticulously evolved magnetic flux within a 1.5Msun star, investigating how fields generated during different evolutionary stages manifest in white dwarfs. Data shows that core-averaged squared radial field strengths, normalized to match observations from recent asteroseismic studies, peak at approximately 100kG within the red giant model. Typical detected field strengths range from around 10kG up to 200kG, with some exceptional detections exceeding 600kG. The team employed the ‘magsplitpy’ code and methods detailed in recent literature, Das et al. (2020), Bugnet et al. (2021), and others, to accurately normalize field configurations and compare them to observed oscillation modes.

These measurements confirm a strong correlation between internal red giant fields and the magnetic properties of their white dwarf descendants. The study highlights a unique shell configuration arising in scenarios where the magnetic field extends beyond the hydrogen-burning shell, compressing the field as it moves outwards. Tests prove that scenarios involving a magnetic field originating in the hydrogen-burning shell can explain the majority of detected magnetic white dwarfs with masses between 0.5M⊙ and 0.64M⊙. Even higher RGB field strengths, reaching up to 600kG as reported by Deheuvels et al. (2023), explain a significant portion of the remaining detections.

Red Giant Interiors Fuel White Dwarf Magnetism, surprisingly

Researchers revisited the ‘fossil field’ framework, investigating how magnetic fields might evolve as a star transitions from a red giant to a white dwarf. This conclusion differs from some previous work, stemming from the use of a stable initial radial profile for the magnetic field, rather than assuming constant magnetic energy. Future research could explore the role of rotational effects and the potential for dynamo action within the radiative interiors of red giants, further refining the understanding of magnetic field evolution.