Is there evidence for field restructuring or decay in accreting magnetic white dwarfs?

TL;DR

This study models how accretion affects the magnetic fields of white dwarfs, suggesting that high accretion rates can significantly restructure and weaken the magnetic field, potentially reaching a minimal 'bottom field' value.

Contribution

It introduces a model for magnetic field evolution in accreting white dwarfs, highlighting conditions under which the field is restructured or remains stable.

Findings

High accretion rates lead to field restructuring and decay.

A critical accretion rate of ~10^{16} g/s influences field evolution.

The polar magnetic field can be reduced to a 'bottom field' of ~10^3 G.

Abstract

The evolution of the magnetic field of an accreting magnetic white dwarf with an initially dipolar field at the surface has been studied for non-spherical accretion under simplifying assumptions. Accretion on to the polar regions tends to advect the field toward the stellar equator which is then buried. This tendency is countered by Ohmic diffusion and magneto-hydrodynamic instabilities. It is argued that if matter is accreted at a rate of $\dot{M}_{\rm crit} \sim 10^{16}$ g s$^{-1}$ and the total mass accreted exceeds a critical value $\Delta M_{\rm crit} \sim 0.1-0.2\ms$, the field may be expected to be restructured, and the polar field to be reduced} reaching a minimum value of $\sim 10^3$ G (the "bottom field") independently of the initial field strength. Below this critical accretion rate, the field diffuses faster than it can be advected, and accretion has little effect on field strength and structure.