Are white dwarf magnetic fields in close binaries generated during common-envelope evolution?

TL;DR

This study tests the common-envelope dynamo hypothesis for white dwarf magnetic field origins using binary population models, finding it insufficient to explain all observational data and suggesting alternative mechanisms may be needed.

Contribution

The paper critically evaluates the common-envelope dynamo model for white dwarf magnetism through population synthesis, highlighting its limitations and proposing the need for revised theories.

Findings

Model explains magnetic fields in polars and pre-polars only after rescaling.

Fails to account for absence of hot magnetic WDs in young WD+M-dwarf binaries.

Predicts too many strongly magnetic WDs in close binaries, conflicting with observations.

Abstract

Understanding the origin of the magnetic fields in white dwarfs (WDs) has been a puzzle for decades. A scenario that has gained considerable attention in the past years assumes that such magnetic fields are generated through a dynamo process during common-envelope evolution. We performed binary population models using an up-to-date version of the BSE code to confront the predictions of this model with observational results. We found that this hypothesis can only explain the observed distribution of WD magnetic fields in polars and pre-polars and the low-temperature WDs in pre-polars if it is re-scaled to fit the observational data. Furthermore, in its present version, the model fails to explain the absence of young close detached WD+M-dwarf binaries harbouring hot magnetic WDs and predicts that the overwhelming majority of WDs in close binaries should be strongly magnetic, which is also in serious conflict with the observations. We conclude that either the common-envelope dynamo scenario needs to be substantially revised or a different mechanism is responsible for the generation of strong WD magnetic fields in close binaries.