Double-faced white dwarfs and the magnetic inhibition of convection

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 $B_{\rm crit}(M,T_{\rm eff})$ required to inhibit convection in both hydrogen and helium for 0.6–1.2 ${\rm M}_{\odot }$ white dwarfs using two methods. Initially, we estimated $B_{\rm crit}\sim \sqrt{8\pi P}$ where P is the pressure at the base of the convection zone, finding that most (three out of four) of the observed magnetic double-faced white dwarfs could potentially be explained by the magnetic inhibition of hydrogen convective energy transfer, with measured $B\gtrsim B_{\rm crit}^{\rm H}$. Then, we incorporated the magnetic field consistently into the stellar structure and directly computed the boundary of convective mixing. With this more appropriate method, we find that only half (two out of four) of the stars could be explained by the magnetic inhibition of helium convection, with $B\gtrsim B_{\rm crit}^{\rm He}$. 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. Given our mixed results, other – including non-magnetic – scenarios should be considered as well.