A Binary Scenario for the Formation of Strongly Magnetized White Dwarfs

TL;DR

This paper proposes a binary interaction scenario where engulfed companions form accretion disks around white dwarfs, amplifying magnetic fields to explain the origin of strongly magnetized white dwarfs and possibly magnetars.

Contribution

It introduces a new binary evolution model involving common envelope phases and accretion disks to account for high magnetic fields in white dwarfs and neutron stars.

Findings

Accretion disks from shredded companions amplify magnetic fields.

The model explains the observed magnetic field strengths in high-field magnetic white dwarfs.

A binary pathway to magnetar formation via core-collapse is proposed.

Abstract

Since their initial discovery, the origin of isolated white dwarfs (WDs) with magnetic fields in excess of $\sim$1 MG has remained a mystery. Recently, the formation of these high-field magnetic WDs has been observationally linked to strong binary interactions incurred during post-main-sequence evolution. Planetary, brown dwarf or stellar companions located within a few AU of main-sequence stars may become engulfed during the primary's expansion off the main sequence. Sufficiently low-mass companions in-spiral inside a common envelope until they are tidally shredded near the natal white dwarf. Formation of an accretion disk from the disrupted companion provides a source of turbulence and shear which act to amplify magnetic fields and transport them to the WD surface. We show that these disk-generated fields explain the observed range of magnetic field strengths for isolated, high-field magnetic WDs. Additionally, we discuss a high-mass binary analogue which generates a strongly-magnetized WD core inside a pre-collapse, massive star. Subsequent core-collapse to a neutron star may produce a magnetar.