The formation of high-field magnetic white dwarfs from common envelopes

TL;DR

This paper proposes that strong magnetic fields in white dwarfs originate from dynamo processes in accretion disks formed during binary interactions involving engulfed companions, explaining observed magnetic field strengths.

Contribution

It introduces a model linking common envelope evolution and disk dynamo action to the formation of high-field magnetic white dwarfs.

Findings

Disk-generated magnetic fields match observed white dwarf field strengths.

Low-mass companions can be shredded and form accretion disks inside common envelopes.

The process may also relate to magnetar field origins.

Abstract

The origin of highly-magnetized white dwarfs has remained a mystery since their initial discovery. Recent observations indicate that the formation of high-field magnetic white dwarfs is intimately related to strong binary interactions during post-main-sequence phases of stellar evolution. If a low-mass companion, such as a planet, brown dwarf, or low-mass star is engulfed by a post-main-sequence giant, the hydrodynamic drag in the envelope of the giant leads to a reduction of the companion's orbit. Sufficiently low-mass companions in-spiral until they are shredded by the strong gravitational tides near the white dwarf core. Subsequent formation of a super-Eddington accretion disk from the disrupted companion inside a common envelope can dramatically amplify magnetic fields via a dynamo. Here, we show that these disk-generated fields are sufficiently strong to explain the observed range of magnetic field strengths for isolated, high-field magnetic white dwarfs. A higher-mass binary analogue may also contribute to the origin of magnetar fields.