On The Evolution of Magnetic White Dwarfs

TL;DR

This study uses radiation magnetohydrodynamics simulations to show that magnetic fields can suppress convection in white dwarf atmospheres at relatively low field strengths, affecting their cooling evolution only at very low temperatures.

Contribution

First to simulate the impact of magnetic fields on white dwarf atmospheres and to analyze how magnetism influences their cooling evolution.

Findings

Magnetic fields below 50 kG inhibit convection in white dwarf atmospheres.

Suppression of convection does not affect cooling rates until Teff drops below 5500 K.

Magnetic white dwarfs are unlikely to show altered cooling rates at observable temperatures.

Abstract

We present the first radiation magnetohydrodynamics simulations of the atmosphere of white dwarf stars. We demonstrate that convective energy transfer is seriously impeded by magnetic fields when the plasma-beta parameter, the thermal to magnetic pressure ratio, becomes smaller than unity. The critical field strength that inhibits convection in the photosphere of white dwarfs is in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG field strengths observed in magnetic white dwarfs, implying that these objects have radiative atmospheres. We have then employed evolutionary models to study the cooling process of high-field magnetic white dwarfs, where convection is entirely suppressed during the full evolution (B > 10 MG). We find that the inhibition of convection has no effect on cooling rates until the effective temperature (Teff) reaches a value of around 5500 K. In this regime, the standard convective sequences start to deviate from the ones without convection owing to the convective coupling between the outer layers and the degenerate reservoir of thermal energy. Since no magnetic white dwarfs are currently known at the low temperatures where this coupling significantly changes the evolution, effects of magnetism on cooling rates are not expected to be observed. This result contrasts with a recent suggestion that magnetic white dwarfs with Teff < 10,000 K cool significantly slower than non-magnetic degenerates.