The Genesis of Magnetic Fields in White Dwarfs

TL;DR

This paper investigates the origin of strong magnetic fields in white dwarfs, proposing that differential rotation during common envelope evolution drives a dynamo mechanism, aligning theoretical predictions with observed magnetic field strengths.

Contribution

It introduces a model linking stellar interactions during common envelope evolution to the generation of magnetic fields in white dwarfs, supported by population synthesis calculations.

Findings

The dynamo mechanism can produce observed magnetic field strengths.

Good correlation between theory and observations for specific CE efficiency parameters.

The model explains magnetic fields in both isolated white dwarfs and magnetic cataclysmic variables.

Abstract

Magnetic fields generated by a dynamo mechanism due to differential rotation during stellar mergers are often proposed as an explanation for the presence of strong fields in certain classes of magnetic stars, including high field magnetic white dwarfs (HFMWDs). In the case of the HFMWDs, the site of the differential rotation has been variously proposed to be the common envelope itself, the massive hot outer regions of a merged degenerate core or an accretion disc formed by a tidally disrupted companion that is subsequently incorporated into a degenerate core. In the present study I explore the possibility that the origin of HFMWDs is consistent with stellar interactions during the common envelope evolution (CEE). In this picture the observed fields are caused by an $\alpha-\Omega$ dynamo driven by differential rotation. The strongest fields would arise when the differential rotation equals the critical break up velocity and would occur from the merging of two stars during CEE or double degenerate (DD) mergers in a post common envelope (CE) stage. Those systems that do not coalesce but emerge from the CE on a close orbit and about to initiate mass transfer will evolve into magnetic cataclysmic variables (MCVs), The population synthesis calculations carried out in this work have shown that the origin of high fields in isolated white dwarfs (WDs) and in WDs in MCVs is consistent with stellar interaction during common envelope evolution. I compare the calculated field strengths to those observed and test the correlation between theory and observation by means of the Kolmogorov--Smirnov (K--S) test and show that the resulting correlation is good for values of the CE energy efficiency parameter, $\alpha{_{\rm{CE}}}$, in the range 0.1--0.3.