📝 SummarXiv

Abstract

About one in five white dwarfs undergoes spectral evolution from a helium atmosphere to hydrogen and then back to helium. These short-lived hydrogen envelopes - the result of residual hydrogen diffusion - are eventually destroyed by either hydrogen or helium convection. An emerging class of double-faced white dwarfs seems to catch this process in the act, with varying amounts of hydrogen across regions of the stellar surface. Here, we quantitatively test the hypothesis that these inhomogeneities are the result of the magnetic inhibition of convection. We compute the critical magnetic field $B_{\rm crit}(M,T_{\rm eff})\sim\sqrt{8\pi P}$ required to inhibit convection in both hydrogen and helium for $0.6-1.2\,M_\odot$ white dwarfs, where $P$ is the pressure at the base of the convection zone. When incorporating the magnetic field consistently into the stellar structure even at high pressures $P>B^2/(8\pi)$, we find that most of the measured magnetic fields $B$ of observed double-faced white dwarfs satisfy $B_{\rm crit}^{\rm H}\lesssim B\lesssim B_{\rm crit}^{\rm He}$, such that the magnetic inhibition of either hydrogen or helium convection could potentially explain the observations. Specifically, order of unity variations in the magnetic field's strength or orientation across the surface could account for the double-faced nature of these stars.