A Short Intense Dynamo at the Onset of Crystallization in White Dwarfs

Abstract The origin of large magnetic fields (≳10 6 G) in isolated white dwarfs is not clear. One possible explanation is that crystallization of the star’s core drives compositional convection, which when combined with the star’s rotation, can drive a dynamo. However, whether convection is efficient enough to explain the large intensity of the observed magnetic fields is still under debate. Recent work has shown that convection in cooling white dwarfs spans two regimes: efficient convection at the onset of crystallization, and thermohaline convection during most of the star’s cooling history. Here, we calculate the properties of crystallization-driven convection for cooling models of several white dwarfs of different masses. We combine mixing-length theory with scalings from magnetorotational convection to estimate the typical magnitude of the convective velocity and induced magnetic field for both scenarios. In the thermohaline regime, we find velocities ∼10 −6 –10 −5 cm s −1 , with fields restricted to ≲ 100 G. However, when convection is efficient, the flow velocity can reach magnitudes of ∼10 2 –10 3 cm s −1 , with fields of ∼10 6 –10 8 G, independent of the star’s rotation rate. Thus, dynamos driven at the onset of crystallization could explain the large intensity magnetic fields measured for single white dwarfs.