Can magnetic fields suppress convection in the atmosphere of cool white dwarfs? A case study on WD2105-820

TL;DR

This study investigates whether magnetic fields in cool white dwarfs suppress atmospheric convection, using spectroscopic data and atmospheric modeling to compare magnetic and non-magnetic stars.

Contribution

It provides observational evidence supporting the suppression of convection in magnetic white dwarf atmospheres, aligning with recent magnetohydrodynamics simulation predictions.

Findings

Magnetic white dwarf WD2105-820 fits better with radiative models.

Non-magnetic stars are consistent with convective models.

Supports the theory that strong magnetic fields inhibit convection.

Abstract

Around 10% of white dwarfs exhibit global magnetic structures with fields ranging from 1 kG to hundreds of MG. Recently, the first radiation magnetohydrodynamics simulations of the atmosphere of white dwarfs showed that convection should be suppressed in their photospheres for magnetic fields with strengths B $\gtrsim$ 50 kG. These predictions are in agreement with our knowledge of stellar physics (e.g. energy transfer in strong magnetic field regions of the solar photosphere), but have yet to be directly confirmed from white dwarf observations. We obtained COS far-UV spectroscopy of the weakly magnetic, hydrogen-atmosphere, white dwarf WD2105-820 and of three additional non-magnetic, convective remnants (all in the $T_{\mathrm{eff}}$ range 9000-11,000 K). We fitted both the COS and the already available optical spectra with convective and radiative atmospheric models. As expected, we find that for two of the non-magnetic comparison stars only convective model fits predict consistent $T_{\mathrm{eff}}$ values from both the optical and the FUV spectra. In contrast, for WD2105-820 only the best fitting radiative model produced consistent results.