Title: Double-faced white dwarfs and the magnetic inhibition of convection Show Chinese title

Title: 双面白矮星和对流的磁抑制

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. Show Chinese abstract

Abstract: 大约五分之一的白矮星会经历从氦大气到氢，然后再回到氦的大气光谱演化过程。 这些短暂存在的氢包层——残留氢扩散的结果——最终会被氢或氦的对流所破坏。 一类新兴的双面白矮星似乎正在捕捉这一过程，其恒星表面不同区域的氢含量各不相同。 在此，我们定量测试了这样一个假设：这些不均匀性是由于磁场抑制对流所致。 我们计算了在氢和氦中抑制对流所需的临界磁场$B_{\rm crit}(M,T_{\rm eff})\sim\sqrt{8\pi P}$，对于$0.6-1.2\,M_\odot$个白矮星而言，其中$P$是对流层底部的压力。 当将磁场一致地纳入恒星结构，即使在高压$P>B^2/(8\pi)$的情况下，我们发现观测到的双面白矮星的大多数磁场$B$满足$B_{\rm crit}^{\rm H}\lesssim B\lesssim B_{\rm crit}^{\rm He}$，这意味着氢或氦对流的磁场抑制可能可以解释这些观测结果。 具体来说，磁场强度或方向在表面上的一阶变化可以解释这些恒星的双面特性。