Multiple channels for the onset of magnetism in isolated white dwarfs

TL;DR

This paper investigates the origins and evolution of magnetic fields in white dwarfs, revealing different mechanisms for high-mass and low-mass stars, and highlighting the role of mergers and internal dynamo processes.

Contribution

It provides new observational evidence and proposes distinct magnetic field generation mechanisms in white dwarfs based on their mass and evolutionary history.

Findings

High-mass white dwarfs often have strong magnetic fields from mergers.

Magnetic fields in low-mass white dwarfs develop gradually over time.

Magnetic field strength increases after core crystallization begins.

Abstract

The presence of a strong magnetic field is a feature common to a significant fraction of degenerate stars, yet little is understood about field origin and evolution. New observational constraints from volume-limited surveys point to a more complex situation than a single mechanism valid for all stars. We show that in high-mass white dwarfs, which are probably the results of mergers, magnetic fields are extremely common and very strong, and appear immediately in the cooling phase. These fields may have been generated by a dynamo active during the merging. Lower mass white dwarfs, which are often the product of single star evolution, are rarely detectably magnetic at birth, but fields appear very slowly, and very weakly, in about a quarter of them. What we may see is an internal field produced in an earlier evolutionary stage that gradually relaxes to the surface from the interior. The frequency and strength of magnetic fields continue to increase to eventually rival those of highly-massive stars, particularly after the stars cool past the start of core crystallisation, an effect that could be responsible for a dynamo mechanism similar to the one that is active in Earth's interior.