Double-faced white dwarfs and the magnetic inhibition of convection

TL;DR

The paper investigates whether magnetic inhibition of convection can account for the surface inhomogeneities observed in the emerging class of double-faced white dwarfs. It first uses a post-processing estimate $B_{ m crit} \sim \sqrt{8\pi P}$ to assess whether observed fields can suppress convection at the base of the convection zone for hydrogen and helium envelopes, finding partial agreement for hydrogen in some high-mass WDs but difficulties for helium. It then implements the Gough-Tayler convective criterion in MESA to compute self-consistent temperature profiles and convective boundaries, revealing a much lower He-inhibition threshold of $\sim 10^5$ G that could explain some cases, while other stars remain inconsistent. Overall, the results show that magnetism may contribute to the double-faced phenomenon in a subset of objects, but a single magnetic-inhibition explanation is not universally applicable and other non-magnetic scenarios remain viable. The study highlights the need for more detailed envelope modeling, including diffusion and mixing processes, and a larger sample of magnetic double-faced WDs to clarify magnetism’s role in surface composition evolution.

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 $0.6-1.2\,M_\odot$ white dwarfs using two methods. Initially, we estimated $B_{\rm crit}\sim\sqrt{8πP}$ 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.

Figures (2)

• Figure 1: Top panel: the mass of the surface convection zone $m_{\rm conv}$ as a function of the white dwarf's effective temperature $T_{\rm eff}$ for several white dwarf masses $M$, and for both hydrogen (H, dot--dashed lines) and helium (He, solid lines) envelopes. We adopt a nominal mixing length $\alpha=1.8$, and also plot $\alpha=0.6$ for the helium envelopes (dotted lines; hydrogen envelopes are less sensitive for the purposes of this study). Bottom panel: the critical magnetic field $B_{\rm crit}=\sqrt{8\upi P}$ required to suppress convective energy transfer, where $P$ is the pressure at the base of the convection zone. Star markers indicate the magnetic double-faced white dwarfs from Moss2025, coloured as the model with the closest mass.
• Figure 2: Top panel: the mass of the helium convection zone as a function of the magnetic field $B$, which is incorporated consistently into the stellar structure using the GoughTayler1966 prescription; see Section \ref{['sec:criterion']} (only some combinations of $B$ and $M$ are plotted to avoid clutter). Bottom panel: for comparison, the critical magnetic field $B_{\rm crit}$ as computed by the simpler post-processing procedure of Section \ref{['sec:Bcrit']} (same as in Fig. \ref{['fig:EnvHeH']}). The thin vertical lines (colour-coded by mass) mark the $T_{\rm eff}$ of the observed magnetic double-faced white dwarfs.