The origin of magnetic fields in white dwarfs remains a fundamental unresolved problem in stellar astrophysics. In particular, the very different fractions of strongly (more than about a megagauss) magnetic white dwarfs in evolutionarily linked populations of close white dwarf binary stars cannot be reproduced by any scenario suggested so far. Strongly magnetic white dwarfs are absent among detached white dwarf binary stars that are younger than approximately a billion years. In contrast, of cataclysmic variables (semi-detached binary star systems that contain a white dwarf) in which the white dwarf accretes from a low-mass star companion, more than a third host a strongly magnetic white dwarf¹. Here we present binary star evolutionary models that include the spin evolution of accreting white dwarfs and crystallization of their cores, as well as magnetic field interactions between the stars. We show that a crystallization- and rotation-driven dynamo similar to those working in planets and low-mass stars² can generate strong magnetic fields in the white dwarfs in cataclysmic variables, which explains their large fraction among the observed population. When the magnetic field generated in the white dwarf connects with that of the secondary star in the binary system, synchronization torques and reduced angular momentum loss cause the binary to detach for a relatively short period of time. The few known strongly magnetic white dwarfs in detached binaries, such as AR Scorpii³, are in this detached phase.

白矮星中磁场的起源仍然是恒星天体物理学中一个基本的未解决问题。尤其是，在进化联系紧密的白矮星双星群中，强磁白矮星（超过约 1 兆高斯）的非常不同的比例不能被迄今为止提出的任何情景再现。在年轻约 10 亿年的分离白矮星双星中不存在强磁性白矮星。相比之下，在白矮星从低质量恒星伴星中吸积而成的巨变变量（包含白矮星的半分离双星系统）中，超过三分之一拥有强磁性白矮星¹. 在这里，我们提出了双星演化模型，包括吸积白矮星的自旋演化及其核心的结晶，以及恒星之间的磁场相互作用。我们表明，类似于在行星和低质量恒星²中工作的结晶和旋转驱动的发电机可以在巨变的白矮星中产生强磁场，这解释了它们在观测到的星群中占很大比例。当白矮星产生的磁场与双星系统中的次星磁场连接时，同步力矩和减少的角动量损失会导致双星在相对较短的时间内脱离。在分离的双星中为数不多的已知强磁性白矮星，例如 AR Scorpii ³, 处于此分离阶段。