Supersensitive seismic magnetometry of white dwarfs

TL;DR

This paper explores using white dwarf pulsations as a highly sensitive method to detect and measure magnetic fields, revealing fields as weak as a few gauss, which advances understanding of white dwarf magnetism and evolution.

Contribution

It introduces a seismic magnetometry technique for white dwarfs that can detect much weaker magnetic fields than traditional spectroscopic methods.

Findings

Upper limits of magnetic fields in 24 pulsating white dwarfs are typically 1-10 kG.

Seismic sensitivity to magnetic fields depends on mode period and surface convective zone depth.

Potential to detect magnetic fields as low as 10-100 G in certain white dwarfs.

Abstract

The origin of magnetic fields in white dwarfs (WDs) remains mysterious. Magnetic WDs are traditionally associated with field strengths $\gtrsim1\,\mathrm{MG}$, set by the sensitivity of typical spectroscopic magnetic field measurements. Informed by recent developments in red giant magnetoasteroseismology, we revisit the use of WD pulsations as a seismic magnetometer. WD pulsations primarily probe near-surface magnetic fields, whose effect on oscillation mode frequencies is to asymmetrize rotational multiplets and, if strong enough, suppress gravity-mode propagation altogether. The sensitivity of seismology to magnetic fields increases strongly with mode period and decreases quickly with the depth of the partial ionization-driven surface convective zone. We place upper limits for magnetic fields in $24$ pulsating WDs: $20$ hydrogen-atmosphere (DAV) and three helium-atmosphere (DBV) carbon-oxygen WDs, and one extremely low-mass (helium-core) pulsator. These bounds are typically $\sim1$-$10\,\mathrm{kG}$, although they can reach down to $\sim10$-$100\,\mathrm{G}$ for DAVs and helium-core WDs in which lower-frequency modes are excited. Seismic magnetometry may enable new insights into the formation and evolution of WD magnetism.